Relive the classic dialogue: "Forget about Freeman!" "They're waiting for you, in the test chamber." "ARGH!"
Black Mesa is a fan-made mod for Half-Life. It recreates the entire game of Half-Life in the latest version of the Source Engine, with vastly superior graphics. Made with the approval of Valve, Black Mesa has been in development since shortly after Half-Life 2 was released in 2004. It's actually been in the making for longer than Half-Life 3 and both of the episode expansions for HL2.
The Black Mesa team have added a caveat however: the game will only go up as far as the Lambda Core level (at the end of which you teleport to the planet Xen). The team is still working on the controversial Xen levels (most fans' least favourite part of the original game) and looking for a way of making them more compelling. Still, even shorn of the game's climax (the highlight of which is the confrontation with a giant mobile bollock spider monster), this is a pretty large game (about 10 hours).
You will require a copy of Half-Life 2 and the original Half-Life to enjoy Black Mesa, with no further payment required.
UPDATE: Black Mesa does not require a copy of HL1 or 2. It just needs the Source SDK, which is included with all downloads of Steam, which is available for free.