According to the game's project lead, Black Mesa is coming out late in 2009. If you're not in-the-know, Black Mesa is a complete overhaul of the original Half-Life created with the Source engine. It's been in the works for over four years and is probably the most ambitious mod ever made. It's coming along nicely.

"The entire first half and then some of the game is completely playable, actually," Project leader Carlos Montero told The Age. There are still a few gaps and holes in the latter portion, but more and more comes online every day,"

The gameplay is "excellent," according to Montero, whose team intends to improve upon the original in more ways than just the visuals. According to Montero, the mod will "minimize the things [that] were hindrances to gameplay and maximize the elements that played really well."  Among those elements: The AI has been completely re-written and other changes made, but Montero won't reveal them until the game comes out to "make sure that players have the sense of awe and excitement when they play the game for themselves the first time, and get to experience it on their own."

I can hardly wait until later in 2009!

Source