AMX Mod is a scripting mod for the administration of Half-Life servers. The mod was initially developed in the pre-Steam era of Half-Life and it has been ported to the steam versions. Half-Life and its mods (Counter-Strike, Day of Defeat, Team Fortress, etc.) are is the game with most dedicated servers for multiplayer games.
Around a half of these servers with an administration mod use AMX Mod, that is around 20,000 servers (of those who log their stats). The beauty of this mod is its flexibility: anyone can make their own scripts for their servers, making it a customized server administration program. Of course, they can do many other things, like implementing games within the game, inside their server. If the person writing the scripts is experienced, he/she can even create his/her own GUI menus for gamers or administrators to use. That gives administrators all the control and power on their server. There are also several add-ons for the most popular Half-Life mods. Really, if you have a server, you have to decide between AMX and MetaMod, and it is only matter of what you need for the server.
AMX Mod X is a versatile Half-Life metamod plugin which is targetted toward server administration.
It has a wide array of scripting capabilities so people can write "plugins", or files which add on to a mod's functionality.
Plugins can take form in administrative services (adding new admin commands), statistics generation (StatsX), fun additions (godmode, etc), gameplay changes (WC3, CSDM), and much, much more!
You can also write modules to expand the functionality of AMX Mod X and add to the scripting language.