For many sites, Adsense remains the best.
But for those kicked out of Adsense or looking for ways to earn via ads, you have other options:
Yahoo! Publishing Network (currently in beta and available to US publishers only)
http://publisher.yahoo.com/
Intellixt
http://vibrantmedia.com/site/web_01a5.htm
Quigo Adsonar
http://www.quigo.com/adsonarexchange.htm
IndustryBrains
http://www.industrybrains.com
Commission Junction Evolution
http://www.cj.com
Kanoodle Brightads
http://www.kanoodle.com/about/brightads.html
Clicksor
http://www.clicksor.com/affiliate_programs.php
Contextweb
http://www.contextweb.com/
Bidclix
http://www.bidclix.com/
Bidvertiser
http://www.bidvertiser.com/
Also consider joining banner ad networks. Advertising networks are organizations that aggregate Web sites that offer advertising space, and sell banner ads (and other advertising options) across them. Traffic requirement is often a minimum of 5,000 monthly impressions, although many do not accept sites hosted by free page services.
Burst Media
http://www.burstmedia.com
Fastclick (ValueClick).
http://www.valueclickmedia.com
Advertising.com.
http://www.advertising.com
Tribal Fusion
http://www.tribalfusion.com