There is a Twitter HUD available for Second Life. Ordinal Malaprop developed it and is making it available for free in-world. You can read more about the HUD at the web site. If you have never Twittered before or have no idea what I”m talking about you can see what it is all about at twitter.com. You can view your own tweets (my tweets) and tweets from friends and people you follow. The end product is something akin to a stream of consciousness for an entire social network.
The Twitter API is open (more here) which allows anyone who is interested to mash and reuse communication through it. LifeHacker.org has some ideas for those curious about what Twitter can be used for and in the comments you can read how others are using it. See where people are twittering from on GeoTwitter, a Twitter – GoogleMap mashup.
The best feature of the TwitterBox HUD (aside from the Twittering) is that it animates your avi with a very twittery particle effect and sound.