Like, you'll put in a name of someone you like and it comes up with suggestions for other people you might like to try? I can't remember the name and can't seem to get it on a search.

Edit: I enjoyed starting the sentence with 'like'.

You mean http://www.gnoosic.com/ ?
Edit: In case you can't find the music map thingy from there here is a direct link: http://www.music-map.com/

That's the one! Thanks for that.

http://www.pandora.com

is also good

You might also want to check out Last.fm.