Yes please, a list would be quite useful. Thanks!
Let me start from the ad networks:
1. Tribalfusion
2. Content.ad
3. CPX interactive
4. Advertising.com (belongs to AOL)
5. Avalanchers
6. Wahoha
7. Knowd (though I don't like it anymore)
8. Adnetwork.net (haven't tried it yet)
9. Hitleap
10. Interclick
11. RealCast media
12. Tacoda
13. Vibrant media (super big guys with different ad units)
14. Hm, guess I can add ValueTech (though I haven't heard them for a while now)
15. Chitika
16. Clicksor
17. InterClick
Now if you have a content-rich website,
I'd suggest you using content discovery networks.
Among good ones, you can get quality traffic at low price from:
1. Nster.com
2. Nrelate.com
3. Linkwithin (though this guys are for bloggers and exchanging)
4. Zemanta
5. Engageya
6. Zergnet (for gaming/movies websites)
7. 2leep
8. Selectornews
9. Visitweb
10. Redtram
11. Outbrain (not sure if you can actually buy traffic from them though)
If you can tell a bit about your website, we can see which of those actually fits you the most.
Different networks have different number of partners in each content category (for example Nrelate works best for entertaining) so you might need to try couple of them before finding the one that fits you.
Have to note that I personally tried only to exchange with Zemanta, so I'm not sure how they work in terms of selling traffic.