What's a mashup and why would you want one?

Great a presentation that finally explains what these are! I was getting some weird looks whenever I brought my potato masher into work.

This is an excellent presentation (albeit it quite long and for those of you with a nervous disposition a slightly scary picture on slide two) which highlights some of the best mashups that are available. I'm not sure about the subtitle though "Do the monster mash"

Thanks to Peter Godwin at Information Literacy meets Web 2.0 for spotting this.