The Mutt E-Mail Client has an interface to call the Mixmaster binary.
Mixlet is a Mix Client in Java. It does not rely on Mixmaster itself to build the messages but rather implements the required procedures itself.
Deacon is developing a Nym-Proxy which lets you use your usual Mail and News clients while still using a Nym. This is for Unixes.
QuickSilver is an actively maintained and developed client for Win32. It doesn't call the Mixmaster binary but instead links against the mix.dll build from Mixmaster's sources. Omnimix is an actively developed and maintained client for Win32. It functions as a news/email proxy enabling users to send anonymous messages using their preferred client applications.
Reliable is a popular piece of remailer software for Microsoft Windows. It is type I (cypherpunk) only but has type II callbacks. So if you additionally install Mix2 or Mix3 on your system it can also do type II.
Mixminion may become the next generation remailing network (Type III). It's not yet ready to provide you with anonymity but it's well on its way.
Free Haven's Anonymity Bibliography has pointers to all the important papers in the anonymity field. It includes recommended reading lists to get new developers up to speed on remailer designs and related systems.
There is a project that makes the nym.alias.net source work with newer PGP
and GnuPG versions. The project web site is at