There's a product called Microsoft Songsmith. You sing something, and songsmith recognizes the pitches and generates a musical accompaniment for them. It's pretty accurate but makes some odd choices sometimes. You can specify what kind of accompaniment you want — techno, jazz, latin, etc. Anyway, some genius put David Lee Roth's vocal (minus backing vocals and instrumentals) from Van Halen's "Running With The Devil" together with a new track of sub-Holiday-Inn lounge piano jazz. This is, no exaggeration, the funniest thing I have ever seen on my computer. I'm laughing even as I'm typing these words and thinking about it.
There are more mash-ups but none of them are this funny. Anyway, have a good weekend.