Unigine is a powerful Graphics Engine (Unigine Corp calls it a Game Engine but it lacks lots of tools and gameplay subsystems to rapidly make a game with Unigine like with Unreal Engine or CryEngine), which is truly cross-platform (only big advanced engine on Linux). They keep releasing Great Visual Graphics Demo + Benchmarks from time to Time like Sanctuary, Tropics, Heaven (image shown above) which stresses even the most pwerful systems. Also the engine is very stable in memory and threading.
There latest demo is Valley (inspired by beauty of nature in Valleys). It contains Mountain region, conifer forests, and weather of that area as opposed to tropical island setting of Crysis 1. IMHO finally this looks better than Crysis 1. I got 27-34 fps on Nvidia Geforce560Ti + Core 2 Duo 2.66 GHz at 1920×1080 Ultra no AA.
Only one word comes to mind BEAUTIFUL.
Download from – http://unigine.com/products/valley/
or see video – http://www.youtube.com/watch?v=drH5fMCISgY