Run. Think. Shoot. Live. Source. Black Mesa is the Half-Life 2 total conversion of the original Half-Life game. Utilizing the Source engine and its endless array of possibilities and powers, Black Mesa will throw you into the world that started the Half-Life continuum and introduced Gordon Freeman to the world as a gun-toting, bullet-dodging Ph.D. in Theoretical Physics.
Black Mesa, after a quarter of a million years in development, has finally become worth the weight. I'm looking forward to the deathmatch and Xen additions, but what they've presented so far is good enough for a 10 in my book any day.