When I'm reading a good book or important article, I sometimes want to jot down notes about a key idea or save a link to the piece for future reference. Bookmarking sites like Delicious are a convenient way to do this for quick website references, but often I want to save a particular piece of information in a way that I'll naturally return to it later on, which may not be very likely for notes in a Delicious bookmark. Plus, if I find the information useful or important, I want to share it widely.
In such cases, I sometimes use Wikipedia as a "personal notebook" of sorts. When I found some interesting studies on crustacean pain, I added a description and footnotes to the article on lobsters. When I noticed that the Wikipedia article on Givewell lacked the most important information from a utilitarian standpoint -- what organizations Givewell has recommended -- I added a section summarizing Givewell's top charities, with some cost-effectiveness statistics. And when I read an interesting perspective on qualia by Gary Drescher, I added it to the page on that topic.
In addition to having social value, this reference-saving system is convenient for me, because the Wikipedia page on a topic is often the first place I go to look something up. The main downside is that writing a note in a format readable by others and integrated into an existing article takes a fair amount longer than jotting down some quick notes.
One aspect of contributing to Wikipedia that I found slightly difficult was learning to use footnotes. The article on citing sources was useful here, as were tools like this one from Google Scholar for automatically generating footnotes in proper bibliographic format.