go to www.blackmesa.com. it's been in the works for ages! A full, fan-based remake of the original Half-Life game is being released at 8:47am New Mexico time, to the world!!!
All you need is a game that uses the Source engine (i.e. Portal, Half-Life 2 etc), or download the 2007 Source SDK and install it, as that is the SDK that Black Mesa uses.
I personally cannot wait till I get to play it! The game is being released in two sections. The first part features every playable level from "Inbound" up to "lamda core" but not including the xen levels, they are all being worked on to make them larger and better than the original.
I'm stoked so much of this community stuff is happening within my generation. It is so fun to be a gamer!