A colleague recently tipped me off to a permanent 'ad' that Wikipedia is running on their site. The leaderboard asks Wikipedia users to to donate money to help keep the site going. I've read that Wikipedia is only staffed by about 30 people and costs approximately $8-10 million to operate annually (most likely due to hosting / server costs for the billions of articles up there).
I like the approach of this message. Help us keep the doors to the site open and we'll reward you by not advertising - ever.