In the Advanced Search (or once you've done a regular search in Google Images), you can change the option from "usage rights: not filtered by license" to "usage rights: free to use or share."
Lists other places that allow Creative Commons searching.
Flickr has collections of photos with different creative commons licenses. You can also filter out copyrighted photos by doing a search on flickr.com, then use the "Any License" dropdown and change it to "All Creative Commons."
Note: Because Flickr, Google, and other search engines rely on web crawlers and user-generated content, there are sometimes errors in the labeling of usage rights. It is your responsibility to do further research if you encounter an object you think may be copyrighted (for example, if it has a watermark or says all rights reserved).