Google Images - Search by Image and Usage Rights
Google Images is a great tool for finding useful photos and graphics on the web.
Did you know that instead of searching for images by keywords, you can search by an image to find similar images? You can either use a photo from the web or upload your own. For more, see this video from Google.
Google has also made it easier to filter your results by the type of usage rights assigned to the image. This is helpful when you want to limit your search to just those images that you can use on your website or presentation.
To filter, run a search then click "Search Tools" on the results page. There is now a drop-down for usage rights. Your options are "not filtered by license", "labeled for reuse", "labeled for commercial reuse", "labeled for reuse with modification", and "labeled for commercial reuse of modification".
For more info on the usage rights filter, see Search Engine Watch.