So far I have (in addition to KotB and RTKoB):
- Little Keep on the Borderlands - this is basically a parody of KotB, and you can use it to supplement/expand the original keep.
Morgansfort and JN1: The Chaotic Caves
Frandor's Keep for the new edition of Hackmaster - stats are a bit different, but some monster swapping and light conversion work takes care of it all. This one is not a parody.
AX3 Capital of the Borderlands for ACKS, which is a practically full blown campaign book.
I like the idea of a MegaKeep next to a MegaDungeon, and KoTB has always been my favorite B/X module (it's the second I played).