let me describe few of them ,.
adsense.com - by google , best among all
adbrite.com - gives all type of ads but not pop up , i love it ,if not using adsense.
there inline ad is very nice .
infolinks.com-inline ads but adbrite gives this function with other type of ads also , therefore adbrite is better..
clicksor.com-pop up , but dont approve your site untill its having a nice traffic therefore you can go for claxon.com for pop ups.
as for as i know , adsense and adbrite should be priority but no dought , many people are not convinced with me
