The OpenXcom website now offers version 1.0 if OpenXcom, a free open-source X-COM clone, recommending that previous versions be uninstalled before installing the release edition, which requires ownership of X-COM: UFO Defense/UFO: Enemy Unknown to play (thanks Slashdot). Here's a version 1.0 feature trailer and here's word on the goals of the project:

The goal of the project is to bring back the tried and true feel of the original with none of the issues. All the same graphics, sound and gameplay with a brand new codebase written from scratch. This should give it:

• Fixability: Play the game natively without any need for emulators or fancy hacks, with none of the limitations and bugs that plagued the original. No more 80-item-limit, personnel limits, funding overflows, disconnected facilities, broken proximity grenades, floating soldiers, etc.
• Moddability: Tweak the game to your heart’s content. Sure the original was pretty good, but maybe you just think it could be that bit better. A nicer base layout, better laser weapons, maybe challenge yourself with a custom game mode, or just put in all the crazy stuff you’ve always wanted! None of it is hardcoded.
• Flexibility: Port the game to any platform you like, customize it to your liking, or use it to make your own far-fetched remakes. The code is fully documented and open-source so anyone can take a crack at it.

For more details check the Wiki.