TwitPub is a marketplace of sorts. You can create an account for free and subscribe to higher quality tweets. Publishers can monetize by having people subscribe to highly focused and well delivered content. It could be anything for horoscopes, to jokes to technical information.
The idea is simple. You start a Twitter feed that has content poeple would pay for. You mark the Twitter feed as private and hook it to TwitPub. When someone wants to follow it, they must pay the monthly price you have established. According to the FAQ on the site, you get at least 80% of the price you set returned to you.
You will have to have excellent content to get anyone to sign up for a reoccurring monthly fee on Twitter. I am interested to see if they are able to monetize this way.