The radierl project implements the RADIUS protocol, as decribed in RFC2865 and RFC2866, as a stack application in Erlang/OTP. It includes test suites, documentation, and an example application that implements basic athentication and accounting servers using mnesia and disk_log respectively.
Ybot is erlang bot software inspired by Github hubot. It supports IRC and XMPP transports and is extensible with plugins. Plugins can be written with Python, Ruby, or shell. It supports IRC chat, XMPP multi user chat, and 37 signals Campfire chat. It can simultaneously run any number of bots on different transports.
ErlyMUD is a rather minimalistic MUD server, written in Erlang and making use of the excellent OTP libraries. The aim is to have solid support for exploration and roleplaying, within a highly fault-tolerant environment where system crashes or reboots are more of an exotic curiosity than a commonplace thing.