It for Windows PCs, or OSX with the proper emulator software.
For a free VTT it looks pretty good, and with less learning needed then MapTools, the leading Free VTT these days.
I'm going to have to give this a proper checking out over the next few days.
It's been added to the Free Resources List on the left side bar.
From the website:
What is TTopRPG?
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).