Soundslice is a service where you can transcribe music easily. Many sites that offer "guitar tabs" use either an ASCII-based format or files for GuitarPro and similar programs. Soundslice allows you to transcribe directly from a YouTube video, so you get the original sound together with the tabs, looping, slowing down and the ability to transcribe songs yourself.

Here's an example: http://www.soundslice.com/tabs/2865/adrian-holovaty-everythi...

Now they've launched a sheet-music player that's featured on the homepage. Adrian has posted an article about it on his blog: http://www.holovaty.com/writing/soundslice-sheet-music/

Currently, only they transcribe the sheet-music, via their Pitch Perfect program: http://www.soundslice.com/pitch-perfect/