Penumbra Fantasy Bestiary (I love these guys, and besides, you can't have too many monster books!)
Player's Guide to the Wilderlands.
All the Kalamar stuff.
But the book I really want to see, that I think will never be published, is Windhaven. It looked really promising, but it is more than a year and a half overdue, if I'm not mistaken, and the website hasn't been updated in about that long.