I work at a hobby shop in my hometown on the weekends. I worked there in college and started back a couple years ago to help me through a rough time (career change) and help them out for the holidays; I stuck around because my discount kicks ass and I get to see the new stuff first. Also, I know my co-workers pretty well at this point, and I like wrenching on stuff and helping newbies out. I also get free stuff sometimes so that's cool. I'm down to more seasonal work now...just one Sunday every month until Christmas, and I get to keep my discount.
I'm a bit of a workaholic...at one point I worked three jobs, seven days a week. I have a really wonderful job now, but I stress about having two days off in a row, back-to-back. If that's your personality, a second job can grant you perspective and a change of pace from your main job, and you might even have a little bit of fun doing it, too.