DEMONOMICON OF IGGWILV: Malcanthet. The Queen of Succubi is detailed, including information on the incubi, her thralls, and so forth.
MULTIPLE DEMENTIA: Thirty demiplanes, some of which are given full treatment (the Demiplane of Imprisonment, the Black Abyss, and Moil) and others simply mentioned in brief.
ARCHOMENTALS: The Princes of Elemental Good are given full statistics and background.
ECOLOGY OF THE KEEPERS: Just in case you'd forgotten about those mysterious pale individuals in black with the goggles from Fiend Folio.
Also, Savage Tidings covers the Sea Wyvern and making modifications to it; Volo's Guide details four outsiders of the Realms (the harmonious choir of the words, the sliver, death devil (jerul), and umbral gloom); Dragonmarks outlines four manifest zones in Khorvaire; Sage Advice is loaded with planar questions; and Class Acts includes variant class features for spellthieves, a guide to the fighter, spells which can't be affected by spell resistance, and a bunch of replacement abilities for clerics in place of turn undead.
Cheers,
Cam