You can have any advertising program as long as they are not in contextual mode, or do not mimic Adsense's program (YPN, for example).
Contextual ads are ads like Adsense, where the company uses a spider to "read" what the content of the page is, and then serve relevant ads about the website.
I've read that it's now allowed even for Contextual ads as long as they do not look like adsense (not mixed with adsense).