I can't recommend anything personally because I've never used any, but..... This is something on the slate for the next version or two. Not giving out any top secret plans but, basically we want to add in some type of builit in payment processing and then then we'd like to add this in as a user feature. One of those things where if an admin wanted to they could charge for it and enable it on a per user basis and the blog would be available in the user's profile.
