PyGCS is a very stripped down MUD-like chat-server written entirely in Python. It has a single "room" and no large database to keep in memory and on disk. PyGCS has no embedded programming language. PyGCS is ideally meant to be a small multi-user real-time chat system for people who have a need to talk to more than one person at a time online. It fits somewhere in between the setup IRC uses (no walking between rooms so to speak) and the MUCK/MUSH/MOO style of server.
Twisted is an event-based framework for Internet applications. It includes a Web server, an SMTP/POP3 server, a telnet server, an SSH server, an IRC server, a DNS server, a generic client/server pair for remote object access (Perspective Broker), and APIs for creating new protocols. It supports integration with GTK+, GTK+ 2, Qt, Tkinter, wxPython, Mac OS X (PyObjC) and Win32 event loops. It also supports TCP, SSL and TLS, UDP, Unix sockets, multicast, and serial ports. Supported protocols include HTTP, FTP, SMTP, POP3, IMAP, TOC, OSCAR (AIM and ICQ), SSH, DNS, IRC, NNTP, Jabber, SOCKSv4, Telnet, SIP (for VoIP), and XML-RPC and SOAP using external packages. Most protocols are supported as both servers and clients.
Benzo is a pseudo-AI bot written in Python. In addition to providing humourous chatter, it performs basic IRC channel maintenance functions. It is written in a modular fashion, so adding commands and functionality is trivially easy. Benzo's 'brain' is a Markov Chain, which uses probabilites gathered in the input text to produce pseudo-random output.
TilkIRClog is a program designed for logging chats on IRC channels, written in Python. It doesn't intend to be an IRC client, so it's fully automated and has many useful features. For example, it can automatically send logs via e-mail, and even compress them. It can also be restarted from a cron job after termination.
Soulseek (http://www.slsknet.org) is a communication and file-sharing system. It is quite similar to Napster: it lets you exchange files, search for files, chat with other users in public and in private, browse their files, and get recommendations for music. SoulSeek for Linux is a GUI client for the system written in Python/wxPython. It includes all the major functionality: chat, searches, and file transfers.