There are many reasons to do this, including replacing system and application icons to create an overall aesthetic, easing the identification of a particular external hard drive by using its product image as an icon, or even replacing poorly-designed icons entirely (we're looking at you, Adobe).
We Love Icons aims to make it easy for icon lovers to discover new icons from around the web that are free for personal use. In their own words, designers Dan Wiersema and Nando Albuquerque started We Love Icons because, well... they love icons! The site itself is very well designed and easy to navigate; there is even an easy-to-spot option for filtering icons based on type: Mac-compatible, PNG file format, or Windows-compatible.
Further adding to the greatness of this site, the How-To page is a great resource for those new to the icon-customization game. There are links to external content on the history of icons, tutorials on changing icons, and even design-related icon articles. The How-To area also features an Apps section which lists several useful programs for replacing and creating icons. Each application list item includes a one-sentence summary, the application's platform (web, Mac, or Windows), and also the application's price.
Overall, I am impressed with the content of the site and the quality of its presentation. We Love Icons has secured a bookmark in my "Icons" folder right next to the link to InterfaceLIFT's icon section. Dear readers, I present this question to you: in my never-ending search for the best icons, do you have any favorites that you have either designed or found? Hit up the comments and share the love.