Alky Project is a DirectX 10 compatibility library by a 19 year old reverse engineer, Brocious, from California, that allows you to play Direct 10 compatible games meant for Windows Vista on Windows Xp. The project is still in pre-alpha stage and allows you to play a few demo games from the DirectX 10 SDK.

“These libraries allow the use of DirectX 10 games on platforms other than Microsoft Vista, and increase hardware compatibility even on Vista, by compiling Geometry Shaders down to native machine code for execution where hardware isn’t capable of running it.”

Which means if you have Windows Vista installed on a low end machine (without Aero Glass interface, etc), you will still be able to run high end games without upgrading your graphics card.
You can download the preview build here

Future releases will support full games on Windows XP as well as Linux and Mac OSX.

While the project looks exciting, it remains to be seen if it turns out to be a vaporware.