Much to my surprise, when I logged into RPGNow this morning I saw that Wizards of the Coast has uploaded the classic Thunder Rift. This amazing little sandbox is one of my all time favorite B/X or BECMI D&D supplements ever released.
They've got the PDF available for $4.99. They've also got Assault on Raven's Ruin for the same price. Hopefully Quest for the Silver Sword and the other products in this line are coming soon.