7AdPower is a relatively new entrant to the popunder broking market. They offer publishers 1.50 euro (about $1.40CPM) for every 1000 popunders that are delivered and remain open for at least 20 seconds. Stats are available online, and updated in real-time. Payments are issued once 50 euro has been ammassed, and are delivered on a NET30 basis.

AdPowerZone are exclusively a popunder broker. Publishers are offered a comfortable $2CPM, with the opportunity to see higher rates when channel-targeted campaigns are sold. In order to be considered valid, the popunder window must remain open for at least 20 seconds. The downside of this validation method is that uninterested visitors are presented with a relatively long period in which to close the window. The upside is that publishers have no conversion ratios to maintain in order to be paid for the exposure. The popunders are frequency capped so as to appear only once/hour. Online, real-time stats are provided to assist in tracking your earnings. Payments are issued NET30 after the minimum payment threshold of $50 has been crossed.
UPDATE (July 5th, 2001): It seems that AdPowerZone have raised their average rate of pay to an impressive $4CPM. Given the promise shown by this firm, such a competitive rate hike gives this program a Recommended rating.
UPDATE (July 14th, 2002): Whoops! During the past year, the crew over at AdPowerZone seem to have dropped the ball to some degree. The company's pop code now launches several windows, and has been known to also spawn download-request prompts for firms such as theftware player Gator. No longer recommended.

AdScholar has built its entire network around CPM-based popup ads. Where these ads differ from those offered by many exit-traffic agencies, or from programs such as PopupTraffic, is in the application of its technology. Instead of popping the ad up in front of the visitor, and breaking their flow of concentration, AdScholar ads pop behind the active browser window, and are minimised to the user's taskbar. Your visitors will only see the ad once it has fully loaded. This technology thus provides both a better user experience and a potentially higher ROI for the advertiser. Ads are shown only once/unique visitor/day. Publishers are offered an impressive fixed rate of $4CPM, and are paid within two months following the conclusion of the period in which their balance exceeds $25. Real-time stats available. Despite running into a few problems lately associated with an upgrade made to their tracking technology, AdScholar remain promising.
UPDATE (May 7th, 2001): While some rumors have cirulated of late suggesting that AdScholar has filed for bankruptcy protection under Chapter 11, Adscholar's Jamie Palmer informed me that only AdScholar Brokers (a sister company of AdScholar Publishers Inc) has been burned by bad debts. AdScholar Publishers still has sufficient cash on hand to continue operations. Read more here.
UPDATE (Dec 29th, 2001): Comments have started to emerge suggesting that this firm, which once held so much promise as to garner it a 'Recommended' rating here, has suffered dearly of late. The most common complaint regards non-payment to AdScholar's affiliated publishers, and I have additionally recently received a complaint from a former AdScholar advertiser regarding their poor (read: non-existent) targeting capabilities, and their determination to include an AdScholar banner on their popunder windows wthout notifying their advertisers about this. New rating: Not recommended.

Bulletmedia launched as a non-exclusive source of banner advertising, but recently shifted their focus away from banners and towards rich-media interstitials, which they serve on a non-exclusive basis, splitting the campaign sale rate with publishers. Don't seem to have their act together. Be wary.

ValueClickMedia (formerly FastClick) is a quality ad network that offers both banner and popunder support for sites on a non-exclusive basis. Their popunder payments hover around the $2.50-$3.00 gross CPM. Popunders are shown once/unique visitor/72 hours, and do not spawn any additional popup windows. Payments are made monthly, within 25 days after the month in which revenue was earned. Recommended.

goClick.com is a reputable pay-for-placement search engine whose affiliate program pays $2.5CPM on PopUnders. Their system supports topical targeting capabilities, and offers publishers with the option of selecting how often the popunder will appear/unique viewer. They pay after $25 has ammassed and accept those international affiliates who are able to accept payments via PayPal. The only drawback with this program is associated with a Netscape bug that they are currently ironing out. Recommended.
UPDATE (August 26th, 2002): GoClick's program evolved into one of the strongest popunder affiliate programs on the net for several months, but has of late declined from that echelon. Chief amongst the concerns voiced by publishers is the tendency of GoClick to slice one's rates to fractions of the base rate (as low as $0.25CPM in some cases) without notification. The frequency with which these rate reductions occur suggests that the program is now worth exploring only as a 'filler' option for most.

PayPopup rewards publishers for the display of their popunders on a raw CPM basis. As such, should multiple popups be displayed to a single user during a single session, the publisher will be paid for both views. Rates vary between $0.35CPM and $1.40CPM, based on the visitor's geographic location. Payment is distributed on a NET-15 basis, bi-monthly, with a minimum payment threshold of $50. Payment may be requested in US or Canadian dollars, and PayPal support is provided. Not perfect, but definitely worth exploring.

Senac.com operates both a low-ball CPM banner program and a $3CPM interstitial program. Both have been criticised heavily, with complaints ranging from spamming on behalf of the network's founder to possibly illegal reselling of inventory from competing ad networks, through to non-responsiveness and non-payment. Not recommended.