DigitalPoints Co-op network, which is actually two-years and 6 days old now, has added new features recently. The first feature is the support to add the nofollow attribute to the links, enabling advertisers to protect themselves from a penalty, if they are worried about it.
The second addition is really a backend thing, where the "weight" calculation now takes into account ad impressions. The DigitalPoint Co-op tracks impressions, so if you have a high trafficked site, your weight should increase over the next couple weeks.
Keep in mind, there are many folks out there that have scrutinized the co-op network and ones link it, so be warned if you want to use it or not.
Forum discussion at DigitalPoint Forums.
