There is YPN which is accessible to the U.S. publishers at the moment There is also MSN program which si going to get launched soon Are there other alternatives to Adsense ? Something with a lot of adevrtisers and paying per click.
There are other PPC programs out there other than AdSense and YPN, but they can't be really considered "alternatives" since they are awful (clicksor, bidvertiser, kanoodle, etc).
There is no dearth of PPC alternatives to Adsense. But as Daniel said, the rates they pay can be ridiculously low, especially fi you have started off with Adsense or YPN. Adsense is without the doubt the best among the lots, with its targetting, which Yahoo still finds difficult. Some of the other PPC's have payment and support problems too. They aren't bad, but you could put them on some new site of yours to see how they perform
I have used bidvertiser didn't pay me much before, but now I heard they pay double the price (more visitors more ppc). The also started an affiliate program Peace,