Kanoodle, Bidvertiser, Clicksor, Adbrite, Alternativeurl, etc etc etc There are loads more which I could mention. All have loads of reviews on here, overall it would be fair to say all have equal people for and against each. Just try them out for a month, see which one works best for you. Although as a starting point, we always say use Adsense/YPN because for most sites they are the best programs to use.
Some PPC bring in fraudulent clicks. I don't want to mention names of some of the companies that do this, but do watch out for this. I'd stick with BIG NAME providers, like Adwords.
Before you Jump into the 2ns and 3rd Tier PPC engines, I would recommend reading this thread over at SEW that discusses people's experience w/ these places and click fraud. Any time I start to think about those cheap clicks I just go back to this thread to remind me why I don't advertise there. http://forums.searchenginewatch.com/showthread.php?t=4001 It's a little dated, but interesting nonetheless.