Windows: I use a lot of Creative Commons licensed images, which I usually find my images directly through Flickr's CC search or by using the CC Search webapp on the Creative Commons web site. CCFinder is a Windows program that adds a few neat tweaks to the image search process.
With CCFinder, you can filter your search to cover CC restrictions such as non-commercial use only or requiring references required to the original author. You can select multiple images from a search and download them to your preferred folder, and also set images as desktop wallpaper.
CCFinder's interface is a little smoother than using Flickr or CC Search directly, but it does have some drawbacks: it only searches Flickr and you have to register the software before you can use it. I'm not bothered by the Flickr-only search as Flickr has the largest collection of CC-licensed images anyway. I'll certainly keep this program on my main computer as I like the interface but it feels as if a developer could make a great Creative Commons search webapp with a similar interface and covering multiple sources.
CCFinder is a free download for Windows XP and above. The $US10 Pro version adds colour filtering.