When you publish videos, you have an option to place it in under a creative commons license instead of a standard YouTube license. Once the creative commons license is placed on a video, it becomes accessible in YouTube's Creative Commons library. ^^ I've put almost all of my videos into their library, and doing so allows other users to freely use my videos in their content. You must own all rights to a video in order to put a creative commons license on it. To change the license, click on the "Edit" button of a video, and switch to the "Advanced settings" tab. Find the "License and rights ownership" section, and there should be a drop-down menu to change the license. ^^
The creative commons is sort of like public domain, only the owners are allowing others to use their content under the "creative commons" license. Public domain only applies to works created by the government (in the USA's case), or works that have had their copyright expire after many years. Creative commons works still have a copyright, but the copyright owner has "given up their rights" to allow others to freely expand upon their work. =)
I make a lot of music videos, so when other people use my music in their video, I look at it like free advertising. I figure people are going to steal it no matter what license I put on it, and it isn't worth my time to fight it. I might as well embrace the free advertising. ^^ Plus, I really like the idea of letting others "build upon" something that I create. I think that is where some of the best ideas can come from, and copyright laws tend to smoother them....
ah yeah that makes sense actually because people always steal music videos, etc. so definitely no point fighting them all and plus if they like the video of yours that somebody else uses then it could still lead to the viewer finding or searching for you.