Project Wonderful: Tiny ads, cheap
The model will be familiar to anyone who's used eBay: You sign up, fund your account (minimum $5 deposit), upload your ad graphic, and choose your site and ad block. Then you specify your maximum bid, when you would like it to start and when you would like it to end. As long as your maximum bid is greater than everyone else's maximum bid for that ad block, your ad runs in the space. But you don't pay your maximum bid, you pay ten cents higher than the next guy's maximum bid. For example, if the current high bidder's maximum bid is $0.40 and you put in a maximum bid of $1.00, you'll be charged $0.50 (ten cents greater than his maximum bid) per day for your ad to run. You can choose when your ad (and your bidding) will start, when it'll end, and the maximum you're willing to spend. If you run out of funds in your account, that's that--they don't automatically charge you anything beyond what you put in or what you specify as your maximum spending.
So why is any of this noteworthy? Like I said, it sounded fun, and it is fun, which is more than I can say for most ad networks. I got to run an ad for my own personal blog on my favorite webcomic's site for, quite literally, pocket change. And I got to create a little ad graphic. Also, the site is well-designed and easy to use. It took me longer to create my 117x30 button (they also do larger sizes for some sites) than it did to set up my account, place a bid, and get my ad up on a site. If you're the competitive type, it could be a bit dangerous--getting the email that says "you've been outbid!" really sparks the competitive spirit, and for some it could be hard to resist dropping $20 into your account and getting into a bidding war. An interesting side effect of the "perpetual auction" is that if even if you get outbid, if the high bidder's bid expires before yours, you can become the high bidder again without doing anything.
As for advertisers, Project Wonderful is currently invite-only. Anyone can buy ads, but sites that want to sell ads will have to fish for an invitation. Currently there are only a few dozen advertisers, most of them in the webcomic biz, but I suspect the network will grow quickly, as might the prices.