This works even with Grooveshark dead!
The awesome people at audiosplitter.fm provide a little code snippet that allows you to get your Grooveshark collection back.
Go to http://grooveshark.com and open your browser devtools, go to the Console tab and type the content of the JS script. You'll get a big JSON blob, if you know how to exploit it good for you, if not audiosplitter.fm can import it, go use it ;)
Great, thanks ! I created a bookmarklet version, with a human-readable output : https://gist.github.com/dlajarretie/e5fb445866781f986e53