SourceMod is a Half-Life 2 modification which focuses on modification scripting, server administration, and plugin writing. Plugins are scripted in the Small language, and allow you to script actions on your server and access the Half-Life 2 (Source) engine with easy functions. If you don't like Small, read below!
SourceMod is completely extensible, and written on the MetaEng API. MetaEng is a system of layered functionality which lets you connect multiple scripting engines to a common interface. The interface lets you expand the scripting engine functionality and easily maintain plugins which connect from the scripting engine to the host application. For example, you could easily write an interface to let you write Perl plugins in addition to Small ones.
You can visit the MetaEng site here.
SourceMod is currently inactive.