Wikipedia is the single most useful web site on the internet. I don't spend a single day without using it (the only other that applies is Google).
Wikimedia (the non-profit organization that runs Wikipedia) lives on donations mostly. This year they would spend $9m and they may not be able to raise that much. Here’s the report for the last year:
They want to keep Wikipedia ad-free so go help here: http://wikimediafoundation.org/wiki/Donate