In Entertainment trading cards, the artist sketch card has become somewhat of a standard. For those of you that don't know, officially licensed, officially printed cards will be sent out to a slew of artists on any given property (some I've done work for include a myriad of Star Wars, Indiana Jones, and Terminator sets). The artist then draws on the blank side of the card with original art pertaining to the series and sends them all back. These cards are then placed directly into packs, not to be reproduced. This means that whichever fan/collector manages to get one in a pack of cards they purchase is the recipient of one of a one of a kind, original piece of artwork. It really is the coolest thing and I would've flipped if these were around when I was a kid and collecting cards.
My latest work on such projects is for Topps, Inc's Star Wars Galactic Files set. Each card has to be approved before artists are allowed to share, but I recently received word that my cards have been cleared. Check out a sampling! My full set is viewable here.