Freedom to Search
Free software may stem from a research mind-set, where shared results lead to a greater good. Free software has built this around various licenses such as GPL and BSD. The ideals behind free software have spread beyond software and can also be seen in other areas: documentation, game mods, illustrations, music - all types of content creation can be licensed in an open and free way.
One problem with all these licenses, as well as the plethora of closed and proprietary offerings makes it hard to find proper, open, material to start from when creating derived works. Here, creative commons enters the picture with CC Search. To quote the website, CC search is:
With CC Search, we give you quick and easy access to the freely licensed content available from many of the world's top content providers.
The metadata attached to our licenses allow content providers easy ways to search within their libraries for licensed content. ...
Recently CC Search has released a new beta version with a new interface. The goal of the redesign was to be able to be able to work with more search result providers.
Even if CC Search gives you a result it is important to double check that the license permits what you have in mind. Here, again, creative commons must be given credit. Their system with signs makes it easy to locate the license that you are after.
Adriaan de Groot (from KDE, NLUUG and FSFE) has created a similar system for common source code licenses. I like to his blog although the proper link seems to be down as I write this.
If you want to search for source code with a given license, try using koderz. The idea seems great, but I have no first hand experience in using the service in any real project (yet).