The undisputed king of mods, as far as games are concerned. No game has near the number of official and non-official major (boasting a large following) mods as Half-Life has.  Half-Life and its Modifications like CounterStrike, Team Fortress, and Day of Defeat have set the tone for the multiplayer shooter genre for years now. Only games like BattleField 1942 and Unreal Tournament 2003 have a chance of equaling that success. Check out the Bot Resources lair for bots that you can use in Half-Life and some of its mods. 

Half-Life Tips
A few tips for the shooter that spawned multiplayer mayhem.

Half-Life 2
The long-awaited sequel to one of the best games ever. Will cater to the mod community even more this time. 

Natural Selection
One of the best and most imaginative Half-Life mods yet. NS plays similar to real time strategy games like StarCraft.

Goldeneye
Total Conversion based on the popular Nintendo 64 game. 

Master Sword
Total Conversion that transforms Half-Life into an RPG. 

Action Half-Life
Mod with an action movie theme.

The Opera
Hong Kong style action. 

Counter-Strike  
Counter-Strike is one of the most popular multiplayer games ever because it built on the overwhelming success of Half-Life and was substantially more strategic than blowing up everything in sight with a rocket launcher.

Day of Defeat  
DoD continues the multiplayer shooter dominance for Half-Life with a World War II theme. Like Counter-Strike, DoD has been so successful that the creators of Half-Life have took an interest in the mod.

Team Fortress
Took team play beyond just the Capture the Flag mode and allowed gamers to assume specialized roles like medics, spies, and demolition man. This mod became so popular that it was eventually included with Half-Life on retail shelves.

Firearms
Added 30 new weapons in this realistic mod that is included with Counter-Strike at retail. 

WorldCraft Level editor that allows you to build your own Half-Life maps.  The Drunken Masters Half-Life clan that runs a server for the Opposing Force mod. A player ranking page shows who is the current frag king. Half-Life: Blue Shift   Official page for the expansion pack. Includes the Opposing Force mod and support for 32-person multiplayer.

WarGamerZ.com  
Besides just running Counter-Strike servers, this site also offers details on some of those frag fests in the Match History section.

CS Gaming
Gamers who want to jump on the the still-impressive Counter-Strike bandwagon are offered a good starting point here with how tos and strategies.

CS Central MapPack
Several Counter-Strike maps with screen shots. There is also an easter egg section where you find either hidden surprises or errors.

Counter-Strike Immortals 
Focused on making newbies better Counter-Strike players, so they wont be easy targets for more experienced CS players. Offers help in areas like tweaking, map strategies, and scripting.