Yahoo Publisher Network. Some prefer it over AdSense for its (supposed) higher CPC, but it doesn't have as large an ad-bank as AdSense, so you may not get as many targetted ads as you would with AdSense.
Adbrite + Kontera should be your second option if your not from the United States. If you have huge traffic then CPM ads would be good aswell.
For me its Adbrite , as this is the only comp frm which i have received some bucks . yet to get anything from google ... have not completed the $100 marks
I think 1st is Adsense then YPN ( if available ) then Adster and then Adbrite. I found Adster rate is nice. and One thing I feel mostly people missing is Parking Domain Companies, Like Sedo and others. some times more than Adsense.
CPC - Adsense - YPN - Adbrite - RevenuePilot - Clicksor CPA - Azoogle - Tribal Fussion CPM - Valueclick