http://publisher.yahoo.com/ Be prepared to wait for a long time, because they really are quite slow in getting back to you. Or just wait until they finish their Beta and apply then.
Be ready for a good wait though . . . One thing I've noticed is that the more unique the niche, the better your chances of getting accepted might be. I don't mean something really quirky, but something informative that hasn't been done to death already, with unique content. I got turned down for more typical types of sites that had more or less voluminous traffic, but got accepted into the program with my esl site. Kind of wierd. So your ringtone site or whatever like that is probably not the best to apply with. Nothing but anecdotal evidence on that, but it's my opinion.