i have heard of obeus offering for pay per impression program..but i myself has never tried it and i have started using it a few days back..so please do a search..but i read it in their terms that they are offering pay per impression for websites..

on my previous blog even google adsense was paying me for impressions..the key thing was if i had 500 impressions of one page and then somebody clicked an ad i used to get 0.5$-2.0$ per click..depending upon the number of impressions..and while in the beginning i was gettin only 1-2 cents per click when the number of page views used to be lesser..

there are many ad services companies being launched which are providing par per impression service and i'll suggest u to do a search for some..