Jaiku is another nanoblogging service, much like Twitter (Webware coverage). It's got a few bells and whistles that Twitter lacks, but the core concept is the same: it's easy to enter in quick status notes; it's easy to subscribe to friends' feeds; and you can send and receive updates by phone.
What I like about Jaiku is that you can add RSS feeds to your account. That way, if you blog something or post a picture to Flickr, for example, your Jaiku feed will automatically share that information with the people who are following you.
Jaiku also allows users to respond to posts in a more straightforward manner than Twitter does. And it supports the updating of your location more elegantly than Twitter (location support in Twitter is a hack developed by the guy who wrote Twittervision).
The problem with Jaiku? Only that everyone is on Twitter instead. But this market is still embryonic. There will likely be several other nanoblog tools released this year, and it's too early for any platform to claim that it's got a defensible share of it.