Sounds like Maptools is exactly what you're looking for. At it's most basic, it allows you to run a server with a map, some tokens to represent enemies and PCs, and an utterly generic terrain map. It has a chat box and has support for dice rolling.
Once you get a hang of it, you can start making your own tokens (with an ancillary program called TokenTool), importing your own maps, implementing fog of war and light, running macros to roll all the dice for your powers, have unique token states to represent conditions, etc.
RPTools - Home
Make sure to check out Tutorials tab for video tutorials and a link to the wiki for more help. There are also forums where you can ask for help if run into problems.
We've been using Maptools for 2 or 3 years now, and I don't think I could possibly ask for anything better.