What's the most appropriate extension for a simple affiliate program?

For the requirements that I posted, this ended up being the most appropriate extension: http://ecommerce.aheadworks.com/magento-extensions/refer-a-friend.html

Customers and affiliates are treated the same way. This requires some manual work for payouts for affiliates

UPDATE: Some things that I've had to customize myself, which would have been nice to be included in the base module:

• Support for customer groups
• The Stop Processing Rules doesn't work like you might expect it to. It stops processing on the rule regardless of whether the rule was applied or not
• Replaced the session variables with cookies for longer-lived tracking.
• Replaced the URL format for referrer URLs with something smaller and simpler.
• Implemented click tracking for referrer URL click-through's

UPDATE 2: Ended up rolling my own. Because of the features mentioned above that I built myself, and also because the database table structure was pretty inconsistent and in a few cases I hit some random cron errors which I didn't get very prompt or useful support responses on.