Boards
Songs that fit together on 'shuffle all' that you never thought'd work. Ever.
I had Radiohead's 'Exit Music (For a Film)' seamlessly bloom into The Mars Volta's 'Viscera Eyes'. It was such a good combination that my mp3 player decided to freeze on choosing the next song. Obviously it thought that it wasn't going to be able to pick a better combination of songs.
Any others worth mentioning?