Counter-Life is a modification for Half-Life that lets you play through the single-player levels of the game with the weapons from Counter-Strike. It seems to be based on beta 4 of CS, because while the HE grenade and Scout are present, the XM 1014 auto-shotgun and Mac 10 (plus every weapon added later) are missing.
Weapons and ammo that would normally be found lying around in the game (apart from the knife, for obvious reasons) have been replaced with piles of cash that can be used to buy guns, ammo, armour, and health at the health and HEV charging stations. The menu system used to buy things is the old text-based one and not the new GUI system, but it works fine (it was broken at first for me, but it turns out I'd misplaced a few files when copying the mod out of the zip file). Surprisingly, the money system doesn't unbalance the game too much - at the beginning, when there's not many items lying around, you can just about afford a Glock 18 and an extra clip of ammo. Later on, though, when you've already got all the weapons you need, it becomes easy to top up on health and armour due to their low cost. When you get to Xen, the game unbalances its self again, as the long jump module disappears due to a bug and the fact that there's no charging stations, making you resort to cheating.
The weapons themselves are also fairly unbalanced - while it takes about two clips from the Commando to kill an alien grunt, the Apache will go down after one hit from the AWP. The game is also fairly friendly with the Fusion packs, if you want to use them. All in all, it's an okay game to play if you're bored and don't mind playing through HL's single player game again.
The Counter-Life site is located at http://cl.cscentral.com/ .