I purchased this extension to use for site where I have several membership categories (free as well as paid memberships). This extension allows users at the front-end to view details various categories, register as a member in a specific category, and pay via Paypal. A user can also upgrade his or her membership based on rules created by the administrator. Auto-reminders at membership expiration is also built in.
The is a very comprehensive, easy-to-use, and clean extension. I give it a 5-stars rating and highly recommend it.