1. AdScholar - are experiencing some bugs in their system while they upgrade to offer better tracking for advertisers. Have been providing poor affiliate support of late, but remain promising. Check back in a few months.
2. GoClick - reputable, offer great payouts and topical targeting capabilities. The only drawback is a Netscape bug that they are currently ironing out. Recommended.
3. PopupTraffic - only counts uniques, but pops up on every page view. A cookie is available to reduce this to one pop/day, which is recommended. Offers popups ($3CPM) and popunders ($2CPM). Recommended.
4. PopupNation - Must be a Websponsors.com member in order to participate. Payment is on a CPA, rather than CPM basis. Worth exploring.
5. PopupMoney - Much the same as PopupTraffic, albeit with lower payouts. Stick with PopupTraffic.
6. Others - AllClicks Exit (accepts new members for a few weeks every quarter, pays a massive $15CPM on exit pops. Recommended). ExitFuel (spawns multiple windows, but is reputable. Worth exploring). Affiliate Target (not worth the trouble - read other threads on this forum for more info). FastClick, Engage, ClearBlueMedia and BlueFishMedia all also offer pop* variants to their publishers, and all are worth exploring.