I normally correct a few grammatical and factual errors here and there, and counter any blatant act of vandalism, though now trying to be more active in the project and moved onto expanding stubs.
I think Wikipedia is a great reference point to initiate and guide in the pursuit of knowledge and I particularly appreciate the project, as it has brought, and is bringing, knowledge and information; hitherto accessible to only a few with the requisite time and resources; within the reach of anyone with an internet connection; quite a remarkable and praiseworthy endeavor, and one I intend to be a part of.
As a 'nerd', I get a great kick out of the fact that, in a rabidly anti-intellectual age, one of the most successful collaborative initiatives and websites is an encyclopedia!
I particularly value the principles of neutrality, verifiability,and worldwide perspective, alongwith all the others of course.