if you haven't tried out openRPG, you might want to give that a look. It has pretty much everything--chat, multi-level maps that give the DM control of visibility and so on, macros for dice rolling (so you can just click "Fireball" and cast your fireball in the chat window, with DC and damage on display), and so on.

If you just want a nice, easy tool for dealing with maps, combat encounters, and that sort of thing on-the-fly and online, try out twiddla or flockdraw. Flockdraw is super slick but the actual canvas is a little small, whereas twiddla is a little clunkier but gives you a canvas as big as your browser window can hold (as far as I know).

I hope one or more of these helps!