PhyziosStudio Pro (Must register a free account at website to use, but it's worth it)
wxsand
EnigmaSand
Powder Game (Updated monthly, must register (free!) to upload (online game))
Pyro Sand 1+2
---- of sand (h-word)
NEW sand (pretty crappy)
Etch-a-Sketch sand (pretty bad too)

I'll update every once in a while.

Incredibots2.com

These are a different breed than powder toy, but still quite fun and simphysicsy:

Stair Dismount / Truck Dismount: Ragdoll abuse, what could be better?

Rigs of Rods: Softbody vehicle simulator. You know how cars sometimes bend when they do stunt jumps (well, landings) in movies? Well, RoR does that and much more!

Orbiter: Space flight simulator. Line up your trajectory, and let inertia and gravity do their thing. Catastrophic failure is ALWAYS an option.

Pontifex / Any other bridge building game: Try to build a bridge strong enough for the train/truck/whatever to cross. Go ahead, I dare you! Or, y'know, you could do the opposite and seek out the most entertaining failure modes.

Roblox: Not strictly a physics game, but has enough to be fun for physics lovers. Hint: LEGO blocks + explosives

dunno if its actually released or not, or if you have to have a massive super computer to run it, but i found this video a while ago. Another member posted it when the pt had the solved and unsolved comment sections before the forums.

http://www.youtube.com/watch?v=dwvukCY5xok

Golems Universal Constructor and Pandemonium

Flight simulators: I love orbiter. Combining that with flightgear and rigs of rods would be lovely. Also X-plane. And Condor Soaring.

Bridge Building Game (like pontifex, only with very simple graphics).

Waiting for Vessel: http://www.strangeloopgames.com/?page_id=3

Um3k
RoR made some progress! I completely forgot about that, thanks for reminding

xspringies.
it simulates masses and springs. it's over a decade old but it is REALLY fast. we've modded it so it can handle some statistics and potentials to simulate primitive "molecules".
actually an awesome program.