Asmor – a number of useful generators, including random encounters and parcel generators.
D&D Insider – lots of new official material, as well as previews of new published material.
Eberron Goblin Translator – for those of you that can’t live without Daask warcries.
HeroMachine – Make your own hero, with this, er, machine.
Shadows from Beyond – An Eberron campaign being run by a player of S.P.I.R.E.
Sharn – Eberron Wiki – a quick primer on Sharn.