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The subscription module from Third Eye is a solid but simple module that generally does what it advertises - it allows manual or automated adding to one or more groups by way of a payment gateway (paypal, ccbill, payflow at this point) at which point their subscription will automatically expire at a set time (and/or automatically renew) and (through a cron job) expires the users and removes them from the group. They have more than one gateway. The free version uses a manual gateway (effectively invoiced, and/or manually addable).
I haven't tried or used Adhesion, yet, but looks to be useful as well.
Good luck!
