I regularly read the SuperHeroHype Boards for discussions on Nolan's upcoming The Dark Knight. I also love looking at the great fan art that people post there.
After lurking for a long time, I thought I'd participate in the thread myself with a go at Ledger's joker. Here are the two versions I posted here:
The portrait was sketched and painted directly in Photoshop and the background images are not mine. I think the joker card is a Bob Kane design.
Click on the images for larger versions.