Apache James
The Apache Java Enterprise Mail Server (a.k.a. Apache James) is a 100% pure Java SMTP and POP3 Mail server and NNTP News server. We have designed James to be a complete and portable enterprise mail engine solution based on currently available open protocols.

DrFTPD
DrFTPD is a Distributed FTP server written in java, it's unique cause it doesn't handle transfers like normal FTP servers. DrFTPD is set up with a master and a collection of file transfer slaves that handle the file transfers, you can have as many file transfer slaves as you like. Some names that could be used to describe this is ftp site merger, ftp cluster, ftp grid or multi site bnc.

JAVA SOCKS Server
It is a SOCKS server written entirely in Java, which supports both SOCKS4 and SOCKS5 protocols. This software is distributed under GNU Lesser General Public License, meaning that both binary and source code are freely available and can be modified an distributed.

JGroups
JGroups is a toolkit for reliable multicast communication.
(Note that this doesn't necessarily mean IP Multicast, JGroups can also use transports such as TCP). It can be used to create groups of processes whose members can send messages to each other.

Netty2
Netty 2 is an event-driven network application framework. It provides an easy event-based API (like Swing) to develop high-performance, maintainable TCP/IP server/client application. Netty handles many essential features such as readiness selection, thread pooling, and buffer reuse which are required to build high- performance and capacity network applications in Java.

QuickServer
QuickServer is an open source Java library/framework for quick creation of robust and multi-threaded, multi-client TCP server applications. With QuickServer one can just concentrate on just the application logic/protocol on hand.