Don't tell them (especially if they are a third-party recruiter) which company. It's not their business which offers you have to evaluate. Recruiters/hiring managers are used to candidates interviewing with and having offers for different companies.
Inform them politely, "I have received an offer from another firm and am required to let them know by 12/1/2012."
You could, depending on where in the interview process you are, ask if there is any way they can move the process faster. Something like "I would really like to consider an offer from Firm A as well. After my last interview, we were both optimistic - is there any opportunity we can help make this happen?" or something similar.
To address some of the comments about "why not to say" (to a company recruiter) -
It is disrespectful to the other company to reveal information made in confidence. How would you feel if Firm B went around telling other companies they had made you an offer?
Would you feel comfortable asking Firm A questions along the lines of, "how many candidates are you interviewing for this position?" or "have you made an offer for this position to another candidate?" (you may actually be comfortable asking these sorts of questions, and if so, imagine the feeling the HR people might have - probably "hm, well, this isn't really your business to know")
This lessens your negotiating ability. If Firm A has no idea what Firm B is, they will not have any information about other offer is, and be forced to adjust their offer accordingly. You want to start negotiations on an offer from a perspective of mutual efforts to find an agreeable deal - this is harder when you add a known third party into the mix.
Everyone involved is a business person. We - as job seekers - tend to make these things way more important than they really are. In all likelihood, if you respond similar to what I've or some of the other answers say, you will not even get any pushback, because the HR person probably will go "ok, didn't answer, oh well" if they even care that much. If they care and give you a harder time for not answering the question I would find that information valuable as well, as it shows that Firm A (or at least the HR people you are dealing with) do not fully respect your privacy in a way most companies will.