Windows: I use a lot of Creative Commons licensed images and usually find my images directly through Flickr's CC search or using the CC Search webapp on the Creative Commons web site. Both of those methods have pluses and minuses but if you need to find CC images daily you may want to consider the Windows program CCFinder.
CCFinder is a creation of German-based developer Abelssoft. It'll run on Windows XP or higher and requires .Net Framework 4. You can filter your search to omit or include various types of CC restrictions such as non-commercial use only, references required to the original author, etc. Once you perform a search you can select a single or multiple images and either download to your preferred folder, visit the source image web site, or set the image as your desktop wallpaper.
In practice CCFinder's interface is a little nicer than using Flickr or CC Search directly, but whether that's worth an entire application depends on how often you need Creative Commons images. I noticed two drawbacks to CCFinder—the program currently only searches for images on Flickr and you must register the software before use by providing your name and email address which signs you up for their email newsletter. I'm not bothered by the Flickr-only search as Flickr has the vast majority of CC-licensed images anyway (you can search Google Images, Fotopedia, Open Clip Art Library, and Pixabay in addition to Flickr using the Creative Commons web site, but you can't search more than one source at a time) and I've only received the one confirmation email in nearly a week of use so I'm not too worried about getting spam emails from the developer.
I'll certainly keep this program on my main computer as I like the interface but it feels like a developer could make a great Creative Commons search webapp that can search multiple sources and keep the friendly layout of CCFinder.
CCFinder is a free download from the developer. After downloading you can donate $10 and receive access to a pro version that adds color filters.
CCFinder: Creative Commons Image Finder | Abelssoft via MakeUseOf