Virtual table tops have been around for a while, with with WOTC's announcement of D&D insider they have hit the main stream.
Savage Worlds have just gone exclusive with one (and not the best from the opinions I have heard), so I though I would give a shout out to the one I think is the best, and certainly has the best options in development.
RpTools (www.rptools.net) have a product called MapTools which is a ambiguous Virtual Table Top (ambiguous meaning it can be used with any system). Trevor the lead developer has a strong voice in the forums and listens closely to the community during development (there is a new build with extra goodies most weeks).
There are 2 versions mentioned on the main page, currently v1.2b32 is the Finalized version and v1.3b14 (at the time of writing, but as I said there are regular updates) is the development version. I personally use the development version, but if you want something slightly more stable use the finalized version of 1.2.
Maptools can be used is several ways, purely as a virtual table top, used in face to face playing instead of floor plans and miniatures. It has an inbuilt dice roller if you choose to use it but it's not really necessary in this scenario.
Or it can be used to bring in players over the net, whether you have one friend who lives a long way away but would like to join in, or to run and entire game this way. Most people who run this way use either skype or Ventrillo for voice, and then use the build in chat system for notes or emotions.
The dice rolling system works, but currently isn't optimal for Shadowrun (you would have to tell it to give you the numbers of every dice rather than just successes and glitches) but there is a request in to change this in one of the upcoming builds.
I could spend lots of time here describing it, and saying what it can do, and miss vital elements. Go download it (as I said it's free) try the development build to see what's happening (it is pretty stable). If you need or want help and advise the forums are very active, and as I said - the developers spend a lot of time there and there are demos run if people ask (there is one today at 8am EST if anyone gets this quick enough). See what you think, it would be a shame to see Catalyst follow the same path as Savage Worlds has because people didn't know what else was out there.
Oh and did I mention, it's FREE!
RpTools Homepage (the home of MapTool)
Cweord
(Same name in the MT forums, feel free to look me up)