Most students at Penn who do work do so via the Work-Study program, which counts towards financial aid. These jobs range in activity from basic dorm mailroom or desk jobs to research assistant positions in Penn labs. From what I have heard from friends, the work-study jobs on campus are pretty good, and although the pay isn't great, it gives students good work experience and helps offset high tuition.
Other students who work have non-conventional jobs--I know one student who is a professional wedding photographer, and another who models part-time. There is also the occasional babysitting job.
That being said, a lot of Penn students don't work during the year, opting instead for summer internships or jobs to make money.
During the summer, many Penn sophomores and juniors participate in On Campus Recruiting (OCR) to secure summer jobs. Most of the kids who participate are in Wharton, because most positions available are in business. However, students in other schools of the university can also get decent summer jobs.