Most job opportunities presented to you by other residents pay nowhere near real-world wages. Sometimes you can probably earn more in a slave factory in Asia. Naturally there are exceptions, but nearly always the most worthwhile job is to work for yourself. If you are ready to invest a lot, buying a sim (or a few) and renting land to others can be very profitable when done properly. You do not want to get into the land business if you don't know exactly what you are doing, though. You can lose quite a lot of money. Also learning how to build, script or create other content can pay off really well, but requires a LOT of excercise and at least some talent.
If you are only looking to have a job for fun, you can consider the sorts of jobs that the others listed. Being a DJ can be very enjoyable if you have it in you, for example, and they are always needed. But I must say, under no circumstances should you try to start lending money to people in Second Life. It is very likely that you won't get paid back, and Linden Lab (the admins) couldn't care less as usual. Actually I'm not sure if moneylending is even legal in SL...