Yesterday's DrupalEasy Quicktip covered 3 modules that can extend Ubercart's functionality. Here's 3 more...
Affiliate 2 - want to build an army of affiliates to bring in additional business? This module helps you quickly and easily set up a full-power multi-level affiliate program. You can set the commission rates for the various levels of your affiliate structure and both you and your affiliates will have access to various commission reports.
UC Views - this incredible useful module exposes some of the most important Ubercart tables via Views. This allows you to build complex views involving cart items, orders, and products. It's a great tool when developing your site.
Secure Pages - this is kind of a no brainer if your users are going to be entering credit card numbers on your site. Any developer worth their salt knows that the site will need an SSL certificate in order to keep everything secure and provide the proper level of trust with their users. This modules allows you to specify which paths on your site need be be served securely (using "https"). It's configuration is dead simple and it is rock-solid when used with Ubercart.
Keep in mind that modules are all in various stages of development (and stability). Your mileage may vary.