Greetings,
Well, that depends on your preferences for advertising formats. Currently, you can probably find a number of Networks supplying:
banners, buttons, text links, popups, popunders, intermediaries, 404 ads.
I'd recommend checking out some review sites to get a better idea of the quality of the partners you'll be dealing with. Adbility.com is quite a good one for reviews.
Farewell,