I currently have three different sites with available ad space.
I have one 125 x 125 banner spot available on BestWebImage.com at $16.95 for one month or $4.95 per week. This site is about improving a websites usability and design. It also gets a lot of Twitter traffic because of my Twitter Toolbar. Order page: http://www.bestwebimage.com/about-bwi/ad-rates/
I have space available on my site MonkeyMeter.com, a road rage site. It's a 120 x 600 sky banner on right side. The cost is $25.00 per 50,000 impressions. Order page: http://www.monkeymeter.com/advertise.php
The third site I get is my polling site, APopularityContest.com. It's a large square banner supporting 125 x 125 up to 300 x 250. There are various rates, but the best is getting 200,000 impressions for $64. Order page: http://www.apopularitycontest.com/advertise.php
If you have any questions let me know!