I use Wikipedia all the time as a starting point. If you know nothing or little about a topic it's the perfect place to get some basic info to lead you in a direction. If I were a teacher though I'd make sure my students know it isn't considered a reliable source (what's printed there today may not be there tomorrow).

It gets a bad rap though. One research paper I read indicated that it was as accurate, word for word, as the Enciclopedia Britanica. An impressive feat (or perhaps a depressing indicator of the quality of the Britanica?) Still, it's best to take everything there with a grain of salt and validate it. I'd hope that any savvy teacher would read the wikki on their student's paper before grading as I'd bet a LOT of students blatantly plagerize from this and other popular web sources.

I had a lot of luck as a student using a variety of media sources. Print, broadcast, interview, etc. With research papers the teacher should be grading the research method in addition to the basics (coherent argument, grammar, etc). Doing all your research on the 'web is a sure way to score low on that segment of the grading.