I like emoticons, it is a way to express my feelings about something through otherwise cold emails or digital letters.
As a software developer applicant, I am wondering if adding emoticons (wisely and prudently) to presentation letters will reduce my possibilities or be seen as a lack of seriousness.
As a hiring senior manager I would suggest not to use them. It's a definite no-no for any kind of corporate environment, and I'd suggest that although they may be accepted in a smaller company or startup, I'd still avoid using them.
Your cover letter is your sales pitch to the company, and it's like the cool software company where everyone rides fixies and develops in their Batman onesies but will still have sales people who dress up a level or more to impress the client.
Your first test is the application you make, you are showing how you interact with customers (in this case your potential employer), I'd always err on the side of making a good impression, leave the emoticons for internal emails once you start.
Frankly, there is no real benefit in adding emoticons and mostly downsides (in terms of looking professional). A cover letter is a professional letter and you should keep it professional.
Whether it is acceptable, truly depends on the job and the industry.
For example - if you were applying for an accounting position, emoticons would be seen as unprofessional. Same for most jobs in finance or retails, for example.
If the job required showing your personality and conveying emotions, for example - a community manager, emoticons may be suitable.
Since, in comments, you are asking specifically about the software industry - again, it depends on the company.
The more corporate the company, the more would emoticons be seen as unprofessional (so, Microsoft, Oracle and such). This may also be the case with smaller companies - say those that deal with the finance industry. If you are joining a small gaming studio, or a startup and they have a culture of "fun" (this is rather subjective), then they may not mind emoticons, though I don't think anyone would see them as adding anything.
Emoticons have by convention been an equivalent to slang on the informality spectrum. Cover letters are more formal than that. For the most part, people will use emoticons when conversing with those they know well or have a reason to develop a close relationship with. "Close" can be manager/employee, but it isn't applicant/recruiter.
Also - emoticons are a well loved modifier because they give an impression of how you are feeling. They soften potentially harsh statements or give emotional emphasis. Cover letters are not generally "soft" and the emphasis should mental, not emotional. Yes, they are cold and formal - that's the point. You don't know the person you are writing to, you don't have a reason to assume a close relationship.
When I do write cover letters and send resumes to people I know well, with whom I would feel remiss if I didn't also share my recognition of our personal connection, then I write a pre-cover letter. My packet is my cover letter, resume and anything else requested, and then the packet (usually these days, it's an email with attachments), has a note on top or in the body of the mail that may have some judicious emotions and definitely warm language. For example, "I really appreciate this opportunity, your vote of confidence means more than I describe" - doesn't really need a ":)" to warm it up. But the PS may be something like "When ever I drink coconut rum, I remember that time of which we shall not speak! ;D" in which case the emoticon is somehow right.
I wish to answer more generally: I love emoticons :)
As you see from other answers and comments, however, some not insignificant amount of people do not like them. :(
Now, what follows is a simple truth that extends far beyond emoticons: saying or doing something that is likely controversial is almost always a bad idea at the beginning of a potential relationsihp, such as with a job application or sales pitch.
Let's take a simple example, using emoticons.
You send me a cover letter with a smiley face. I like this sort of thing, as frankly I take some pride in being part of the generation that made them "a thing" (in my day, we didn't have MySpace and Facebook, we made our own online communities! spit) and having myself toyed with them for hours with friends during my youth. As much as I like them, however, at most I consider them neutral for consideration of a resume.
This bears repeating: at best, the result will be neutral to very mildly positive, at best.
Now, this same cover letter goes to someone else. Unless they are just big fans of a smiley face, the best you can probably hope for is neutral. Everyone else will consider it a negative, and I think I can safely say no one gets hired because of a well placed smiley.
This general advice applies to jokes, slick cultural references, using weird fonts, using informal slag, text shorthand, leat speak, etc. If you don't know your audience, you are almost always better off to stick to things that are much less likely to engage someone's mostly irrational opinion.
Know what I mean, Vern? ;)
Finally, one more note:
Emoticons are often used to try to resolve ambiguety. Consider this sentence:
"If I have to deal with another situation like that I'll shoot myself!"
If this was your sentence, you'd probably think you need to put an emoticon after just to make it clear you are joking, right? This means the sentance can be taken a good way or a bad way. In formal writing, this is a huge red flag: there is serious ambiguety in the written communication, and you need to fix that right now!
In formal writing you should use the urge to put in an emoticon as a "FIX THIS" warning from your subconscious. Fix the ambiguity, such as simply be realizing that sarcasm doesn't go well in professional, written contexts.
In short, the answer is no, because it is inappropriate for the context. But let's address the core issue: What are you hoping the emoticons will provide?
Permit me, unmet friend, to assure you that prose can do anything an emoticon can do.
If you perceive e-mail as cold, practice conveying warmth in the text of your e-mails.
If you think the recipient will fail to receive accurate information about your emotions, practice conveying that information with words.
If you want to show off a unique and quirky personality, show it with writing that is professional but ever so gently off-beat. Aim for human, not for strange.
If all of this sounds difficult, practice in this way: Smile as you type. Imagine yourself giving a warm handshake to the recipient, and type your e-mail or cover letter in that frame of mind.
If you have emotions that must be communicated, state them, but measure them carefully and with a sense of priority and purpose. Emphasize the most important emotion and let the bulk of unemotional text that surrounds it give it weight. If you are excited to be applying for a job opportunity, say so (and say why!) then return to talking about your eminent qualifications for the position.
People often complain that pure text lacks tone, and I have been guilty of parroting that prejudice myself. But I do not believe it. I believe that text lacks tone when the writer does not cultivate a proper tone. I believe that words fall flat when the writer is doing nothing to keep them aloft. I believe that professional writing can be warm and convivial while suffering no loss of seriousness.
And I believe that you can write in this way, if you are willing.
In any formal writing, with the possible exception of a formal paper on emoticons or txtspk, emoticons are going to come over as immature and lazy. Even in the minority of cases where it won't harm you, it's unlikely to benefit you either.
For the majority of employers, you might as well write "LOL! I won't take this job seriously if you give it to me LOL!!!!111!!11!one!!one!!!1", enclose a photo of your genetalia with the letter and be done with it because you will stand as much chance of being hired either way.
Sorry if the paragraph above seems overly harsh or rude, that isn't my intention, but I do think its important to outline how much of a faux pas I think this would be.
Your cover letter is not the place to express your inner child. So in other words you should leave the fancy fonts, wild and vivid colours, and smiley faces out and keep it professional. As a general rule, your uniqueness should be expressed through your job skills and history, not through the use of flowery stationery, or emoticons such as smiley faces, frowns, and winks.
Hiring managers are accustomed to seeing proper English language and punctuation. For example. If a applicant for a financial administrator sends in a cover letter on pink stationery with emoticons, the hiring manager will think, "if this is how they communicate with our high-powered clients, it will undermine our company's credibility." Keep in mind how you will feel if you were the hiring manager and you saw a professional document slide across your desk decorated in smiley faces.
;)very, very carefully.