Think I already mentioned it before, but Unity3D is an engine/platform you can use to design games/sims. Cool features include a web based plugin, MS/Mono support, Wii and iPhone dev, and apparently MMO dev in their 2.1 release. Basically, it's in the same niche as Torque X. My main concern remains development tools (ie, art and assets, not game code), but it looks like they're both coming along nicely.
I'm not ready to spend money on either one yet as I'd prefer to get a little more experience with the fundamentals (namely, SDL/OpenGL), but I'm kind of leaning towards Unity atm.