Sorry, but I can't agree with @kolossus' answer.
In fact I prefer working with people who have various interests. If someone is only focused on a single thing, this may mean that:
- They are unable to spend their time effectively;
- When they feel frustrated (everyone has such moments), they will be unable to switch to another activity;
- They will be unable to re-use their expertise from another activity, since the most prominent ideas usually appear on the edge between two areas of knowledge;
And yes, one of my favorite questions during an interview is "what are your other interests?"
I recall when I had a junior developer who was also a prominent video operator and video engineer. He used to be a student in cinematography academia. He worked less hours a day than the others because he needed to attend his cinematography classes, but his real output was much better than the others'.
Assuming that the project you participated in was relevant to a position you're applying to, you should not hesitate mentioning it in your résumé.
You may simply skip the employer's company name, but you should be ready to provide with references when asked.
Alternatively, you may have two sections in your résumé:
- Projects, where you list all necessary technical details (development tools, platforms, your role in very this project, etc.)
- Career, where all employers and formal positions listed.