Rotator-Twister is a memebership system that allows users to create rotators.
A rotator gives them 1 URL at your site that 'rotates' between a list of sites they want to show to others. Users can control 'how' the sites are shown (which are shown more often), and can see statistics about historical views, referring URLs, and so forth.
There are no credits here, just rotation and good historical statistics.
Upgrades are available to get more rotators, or to allow more sites per rotator. Anyone viewing the rotator can pull down a list of URLs. This is popular with traffic exchange owners, so they know what sites will be shown through your rotator.