If you use Twitter and a Macintosh computer you should check out Twitterrific. The new version is nice and the ad revenue / registration model is well done. You can buy the application or allow it to drop an advertisement into the stream once an hour.
I happily paid for my copy. Not because I minded the ads (they're no big deal), but because I think it's easily worth $15 to me. I do wish Twitter got a bit of that, but maybe I'll just get Alex a drink someday.