Other Ad providers along with Adsense?

[markfromslo]

I run Adsense on a high traffic political site (a real site, not just an Adsense site). Get's thousands to tens of thousands visitors a day. The Adsense ads are display type. Am I allowed to run other display advertisers, or is that a violation of Google's terms? (I know they have some problem with other text ads, right?)

If it's okay to run other advertisements, what are some recommendations for other Ad providers that I can add display ads from?