AMX Mod X is a Metamod plugin which allows you to write modifications for Half-Life with Small scripting language. It provides in-game administration tools, game statistics, server management and gameplay modificators. It is based on the original AMX Mod.
We've cleaned up a few things, but for the most part it's not vitally necessary that you upgrade.
The big notable changes: We've split the SQL/DBI modules off into a separate package called "SQL Modules". MySQL up to 4.0 support is still included in the default install, but for 4.1 support or MS/PG/SQLite support, you will need to download the SQL Modules package.
Website: http://www.amxmodx.org/
Full changelog: http://www.amxmodx.org/forums/viewtopic.php?t=11751
Download: http://sourceforge.net/project/showfiles.php?group_id=100470