I rely a lot on the famfamfam.com Silk icon set for use in my projects because the collection comprises of an array of open source icons to fit almost any purpose. Like others, I have trouble browsing the icons to find what I need.
So, I made my own JavaScript famfamfam icon explorer. I know this isn't the first one about, but I wouldn't class it as a bad addition to the bunch. It allows you to quickly search and download the icons just by clicking on them, as some of the explorers I came across didn't have this basic functionality.
Below is the web version and a copy zipped up with the set for local use.
Web version: http://aelabs.net/famfamfam/
Zipped with the set: famfamfam_silk_explorer.zip