The zdoom/gzdoom port has a ton of programmability including the original ACS (action code script) from Hexen and a newer scripting language called zscript. It

The zdoom/gzdoom port has a ton of programmability including the original ACS (action code script) from Hexen and a newer scripting language called zscript. It's basically a fully scriptable doom engine--enemies/actors, events in the level, etc. can all do custom stuff.