I added a small Google AdWords block to the top-right of the wiki. I did this because 1) the extra money would be nice, and 2) I had an AdWords account sitting idle. It seems to blend in well and should be relatively unintrusive, but if you have any suggestions, let me know. Today's Earnings: $0.32 Woo! I'm almost up to a free stamp!
If you are interested in doing something similar on your MediaWiki site, I wrote a short blurb about how I did it.
(Ironically, most of the ads I've seen so far has been for Knoppix and Xandros.)