Apache SpamAssassin is an extensible email filter that is used to identify spam. Once identified, the mail can then be optionally tagged as spam for later filtering. It provides a command line tool to perform filtering, a client-server system to filter large volumes of mail, and Mail::SpamAssassin, a set of Perl modules allowing Apache SpamAssassin to be used in a wide variety of email systems.
OpenDMTP (Open Device Monitoring and Tracking Protocol) is a highly configurable and extensible protocol for communicating with mobile devices over high-latency/low-bandwidth networks. The protocol is particularly geared towards the transmission of GPS base location information and includes a full-featured reference implementation showcasing its capabilities.
Echomine Feridian is an easy-to-use Java API that gives you quick and easy access to the XMPP network used in IM services such as Google Talk. The API allows you to communicate with Jabber/XMPP servers for sending and receiving instant messages, presence management, and custom extensions to the XMPP protocol.
SubEthaSMTP is an easy to understand Java library that provides a receptive SMTP server component. By plugging this component into your Java application, you can easily receive SMTP mail using a simple abstract Java interface. Also included is a small sub-project called Wiser, an easy to use incoming mail testing framework.
The Darwin Calendar Server is a standards-compliant server that allows multiple users to collaboratively share calendaring information. It provides a shared location on the network to store schedules, and allows users to send each other and manage invitations. In order to provide interoperability with multiple calendaring clients, the server implements the CalDAV protocol, which is an extension of WebDAV, which is in turn an extension of HTTP.