The first place to look is the Mambo Code forge. The forge supersedes the MamboXchange and marks a new step in the development of the Mambo community.  Launched in September 2007, the forge is still new, with new projects being added all the time, but the advantage of this is that the extensions on it are up-to-date and working with the latest Mambo releases.

You can find the Mambo Code forge here: http://mambo-code.org/

You can also find a range of extensions on 3rd party sites. These are sites that are not part of the Mambo family of websites (ie. unofficial and not endorsed by the Mambo Foundation, Inc).  http://mambo-extensions.com/ is one of these sites.