Quassel IRC is a modern, cross-platform, distributed IRC client, meaning that one or more clients can attach to and detach from a central core, much like the popular combination of screen and a text-based IRC client, but graphical. In addition to this unique feature, it aims to be a comfortable chatting program.
Agora is a Web-based, threaded discussion tool, similar to Usenet newsgroups. Agora remembers each user's preferences, and allows threaded, unread-only browsing. It offers the ability to manage several different groups, with different themes, forums, and administrators for each group. Agora offers a basic Web-based ICQ client, as well as a Web-based notes system.
ARSC Really Simple Chat is a Web chat system that uses PHP and MySQL. It works with every browser in existence, even text-based ones, and it uses the server push technique or, if possible, its own socket server. It is very simple to install and use ARSC. It includes some important features, such as operators, kick-ing, whispering, and more. Multiple rooms and different languages are available. To run ARSC on your site, you need a Web server that understands PHP, and can connect to a MySQL database.
SurrealServices is a full replacement IRC Services package providing NickServ, ChanServ, BotServ, MemoServ, and OperServ, as well as LogServ and ConnectServ. Features are loosely based on ConferenceRoom/Auspice and Anope, as well as NeoStats. Some security features are also available such as a TOR (http://tor.eff.org/) detector/banner and a country detector (which checks IP allocation and optionally bans). It is multithreaded to eliminate lag due to the asynchronous nature of MySQL and DNS lookups.
Globe*Trotter is a guestmap. It allows Web site visitors to mark where they live on a map and include their personal comments, instead of merely tacking their personal regards onto the end of a long list. It benefits your visitors because they can easily read other's comments and add their remarks in a fun and novel way. It also benefits you, the webmaster, because more people will be likely to give you feedback and you will be able to see where your visitors are. A free hosted version of Globe*Trotter (with no ads) is available.