Alternatives to google adsense: http://www.internet-search-engines-faq.com/google-adsense-alternatives.shtml
Google has created the ability to load an alternate ad when it cannot find a matching ad. This is accomplished by setting a variable called google_alternate_ad_url in your AdSense layout code: google_alternate_ad_url = "/adsense-alternate-ad.shtml"; These alternate ads can include advertisements from Google AdSense competitors. Unless I'm totally misunderstood this is not the case. Using a competitor (using conceptual advertising to show ads) to adsense on the same page is considered a violation of the TOS and might get your account banned.
http://www.bidvertiser.com/bdv/BidVertiser/bdv_publisher_guideline.dbm They seem to making a go of it.