Let's see... in the Eberron game that just ended, if a monster wasn't in the MM, it was probably from the MMIII or the Eberron campaign setting (like dolgrims, for example), although I did use the Fiend Folio and Tome of Horrors II a few times, and even the Advanced Bestiary once (war machine skum nearly resulted in a TPK of 5 6th level players, although it was CR 7. Thanks, Advanced Bestiary!).
In my upcoming Planescape game, I think I'll be seeing a lot more use out of Fiend Folio than MMIII, and a lot of use from the wonderful conversions of monsters I've found on the web (like Pants' fiends project).
Demiurge out.