CJ, Websponsors.com, DirectLeads, OnResponse and Sponsorships.net all offer quality pay-per-lead and pay-per-sale programs that could prove profitable if well suited to the audience of your member sites. Some of these offer their own banner rotation system, while others can be plugged into your own rotation software (AdCyle, AdJuggler, WebAdverts, Central Ad, PHPads).
Finding the right combination of ads to rotate will take a little bit of trial and effort, but may net you $0.30+ effective CPM if you strike it right.
You will, though, have to check each company's terms and conditions before rolling out the rotation over your member's sites, as some may not permit this.
BTW, all of the programs listed above are reliable and do pay. Since so many advertisers are still looking to pay according to measurable results, the CPA networks offer far more stable and strong payouts right now than many CPM networks.