Upgrades
This plugin is a big one as it allows users to purchase credits (through seamless paypal integration), and subscribe to packages enable extra features.
For example, you can use it to allow users to buy more space, more forums or simply make certain plugins available only to users who hav ethe credits to purchase them.
And, as if that wasn't enough, you can gift credits to users, or all users. Set amounts of credits to be automatically given to new users and allow users to gift each other credits.
And of course, it works in any currency you care to mention!
It's inspired by the wordpress.com upgrades system.