I have a handful of websites that do well with AdSense. 95% of traffic comes from a double opt-in mailing list, and I send about 80,000 emails a month to these subscribers. The only links I've ever put in the emails are ones to our sites as we update them. I'm thinking, though, that these mailings themselves could be monetized, but I am not sure how to do it. Maybe an affiliate program of some type? Or approach advertisers directly to see if they'd pay to have a link to their site placed in the emails?