If you freelance on the side, then at some point, you have to decide if you're going to tell your employer or keep it on the down-low. Sometimes telling your employer can yield useful contacts. Or it could give the impression that you'll jump ship at any moment to freelance full time. Of course, if you keep it quiet, it's also possible that your secret could back-fire if someone finds out, particularly if your company has a no-moonlighting policy.
In my case, my boss knew about my freelance writing when he hired me. In fact, I leveraged my blog and writing clips to set myself apart. But I wasn't too public with other people in the company, because freelancing is the kind of thing that can be hard to explain ("what is it that you do? And you actually get paid for that?"). I also wanted to avoid the stigma of appearing disloyal, so I mostly kept it to myself. Well, one day my photo appeared in The Boston Globe and the article mentioned that I freelance. It gave me a ton of great exposure, but it also outed me to my coworkers. At that point I didn't have immediate plans to go full time, but within four months, I had given my notice.
For those who are still working at a day job, do you tell your boss about freelance projects? What about those of you who've left the 9-5 world? How did you handle both worlds?