Wikipedia

I love Wikipedia! It's great for simple definitions, basic information about people, and overviews of topics. I know it's not 100% reliable and can't always be trusted, but that doesn't make it useless. As part of my cataloguing job, I've begun creating name authorities and Wikipedia is a fully accepted source of information for these, for instance, this is my citation of it to justify adding a date of birth for Hicham El Guerrouj:

100	1_ |a El Guerrouj, Hicham, |d 1974-
400	1_ |a Guerrouj, Hicham El, |d 1974-
670	__ |a Moroccan success, c2009: |b t.p. (Hicham El Guerrouj)
670	__ |a Wikipedia, Mar. 9, 2010 |b (Hicham El Guerrouj; b. Sept. 14, 1974, Berkane; Moroccan former middle distance runner)

It's excellent for putting things concisely in a form that can be readily transferred to an authority record. It's also useful for explaining concepts that need subject headings; a broad overview of a topic is often enough to suggest sufficient synonyms to find suitable headings. I'll say it again: I love it!