Inclick from
http://www.inmotiongroup.com/
Script is free to download and setup ... but they take a cut of your revenue ( 15% i think )... actually a very good script IMO
----- searchfeed etc can be used as filler so there are always ads and revenue .... nice touch i think
www.walkerwebsites.com also do one ( bannner exchange & text ad exchange type of script ) .... its ok ... at the best .
There was a sweet adbrite clone sold recently on sitepoint ... the seller may advise you better ... but it was the business ....
http://www.sitepoint.com/marketplace/auction/2474?
Ger
