I like to use a number of tools to evaluate usability, including basic strategies like heuristic evaluation, crowd-sourced tools like UserTesting.com, powerful tracking suites like Google Analytics, and A/B testing tools like Google Website Optimizer and Optimizely.com.
These kinds of resources help test everything from entirely new products to details as small as button labels, and are crucial to running a data-informed business. Small changes can create huge improvements if tested correctly; the key is figuring out what kind of test will provide the information you need early enough to make a difference (and don't forget about statistical significance!)
Here is a list of additional tools to check out; there are many others:
18 Useful Web Usability Testing Tools