TTopRPG is a free virtual tabletop that can be used to play various RPG's online using a PC with XP or Vista (or a Mac running a Windows emulator - 2 button mouse HIGHLY recommended). I recently heard it also runs well on Windows 7.
I designed it around the minimal utilities I considered necessary to run an RPG game. It isn't yet in an officially released form, as I continue to tweak, rearrange and add new functions as I find them necessary. I primarily use it to run DnD 3.5 games. It can also run 4e games and supports most nWoD die rolls (although other aspects of the nWoD system are not supported). As of last estimation, it seems to have 50 or so users, some worldwide.
Here's what the running application looks like from the DM's side. This screenshot uses the map from Paizo's free GameMastery module D0 - Hollow's Last hope. The PC portraits are the iconics they featured in their first Pathfinder Adventure Path, Rise of the Runelords.
The quick instructions: Download and run the two required updates below. Then, a little farther down, you'll find a zip file to extract into its own folder.
TTopRPG 2.0 runs using the .NET 3.5 framework and was developed using Visual C++ 2008 Express Edition. It's expected to run on Windows XP SP2 and Vista (no support for Windows 2000 at this time, even SP4). Currently, it uses GDI+ to render its images, but I'll be updating eventually to OpenGL or DirectX.
Don't have Service Pack 2 for Windows XP? Get it here.
It is apparently also possible to get this application working on Mac OS X 10.5.4 using the Parallels virtual Windows environment. I haven't seen this myself, but the user said it works well. So, bonus! However, one user reported that ctrl key and right hold functions don't work (such as ctrl selecting multiple icons and right pressing to erase sketches). So I highly recommend you get a 2-button mouse for your system if one is available (although I don't know if you can disable the system's capture of the ctrl key for second mouse button use).
Note: TTop also works with VMWare on a Mac. To host, you may need to set the network between the Mac OS and Windows OS to 'bridged' instead of using NAT.
Caution: I've had one report of an XP machine that still failed to run my app even after running the 2008 Redist and .NET 3.5 installs. I have also read reports of other machines having more severe problems attempting to install the packages. I assume these circumstances are rare, but I thought it only fair to warn you. As with anything, installing these are at your own risk. But they should be reasonably safe, since Microsoft provides them with the intent to update XP and Vista.