sxmppd is a server for the XMPP (Jabber) chat protocol. While other, functional XMPP servers exist, they are not as simple or have many dependencies. It supports user authentication, SSL, roster management, presence updates and subscriptions, messaging (local users), group chat, and file-based server configuration.
tkbiff allows arbitrary commands to be executed upon mail reception. If you like programs such as xbiff and xbiff++ but wish they were more flexible, then you'll like tkbiff. Unlike other biffs, tkbiff is fully customizable. tkbiff also doesn't waste your valuable screen space with icons; instead, it shows you the mail itself. It supports UNIX, Mac, and Windows, IMAP, POP, and UNIX-style mail files, and SSL and APOP.
Tmpf is tiny script (written in Ruby) to write standard output to a temporary file, run an application with the name of that file as an argument, then delete the temporary file when done. This is useful when you want to pipe output to a program that only accepts files, not standard input. Example: cat myfile.ps | tmpf gv (equivalent to: gv myfile.ps). Despite the incredible simplicity of this script, there is nothing quite like it provided among the standard Unix tools.