The 23C3 offered a lot of excellent and inspiring talks and the official recording will hopefully be online soon. One talk (german) about Jabber/XMPP reminded me on a Chaos Radio broadcast (german, too) about the same topic that I heard recently. As it seems that not too many people know about Jabber/XMPP I would like to propagate it here.
The Extensible Messaging and Presence Protocol (XMPP) is mainly for instant messaging but but has much more potential as it is an open standard and a design that makes it easy to extended it.
I started to use Jabber years ago but most people I wanted to chat with had already other (proprietary) instant massaging solution like ICQ. So they did not want to register to another IM service (actually this is much easier in Jabber). Luckily Google uses now XMPP for Google Talk. This means everybody with a Google mail account can use and be contacted via a jabber client and the amount of jabber users is increased the dramatically.
Among other functions it is possible to use XMPP for VoIP (Jingle) and or RSS-like notifying (PubSub). There are a lot of clients for many platforms. As it is easy to extent and libraries for nearly every programming language are available you might be able to hack another useful service with XMPP.
If you like instant messaging give Jabber a try. For a quick introduction have a look the Wikipedia page about Jabber and find more information on the Jabber homepage.