Mashups!

A mashup is a mix of different songs to make a new song. http://www.youtube.com/watch?v=zLDZ_zfQ2fM&feature=related and http://www.youtube.com/watch?v=g_AklBD1E_c&feature=related I don't know any good mashups... here are two interesting ones I found :)
What makes a mashup good is when the different songs blend together like a real song. What makes a mashup bad, is when you don't blend the songs together, and you can tell that something is isn't right. Sadly, I didn't find any sites with tips :(
People who make mashups are true musicians. They have to know the beat of a song to match in with another.