We've definitely caught the Twitter wave here at TUAW (we even have our own account), but it is by no means the only 'what are you doing' or lifestream service on the block. Another big player is Jaiku, which brings a fairly different perspective and set of features to the table. While Twitter focuses primarily on asking 'what are you doing?' and offers straight text input, Jaiku allows you to add RSS feeds from virtually any other blogs, sites or communities you post content, generating a sort of play-by-play for almost everything you're creating on the web. This definitely swings Jaiku over to the lifestream category, and it has a number of other features that will likely appeal to those who are looking for more than a plain text mini-blog, such as 'channels' that allow Jaiku users to all post in one theme-specific area (likening it to a giant chat room), as well as an actual comment system on posts.
A major boost for web services like Jaiku (where we also have an account), in my opinion, is great software that can hook into them. While using a browser to interact with these services is fun and all, nothing beats the speed, flexibility and (ideally) extensibility of a well-written desktop client, and Juhu for Jaiku is exactly what I'm talking about. Doing for Jaiku what Twitterrific does for Twitter, Juhu is an app that lives in the menubar, giving you a resizable window for peeking in on what your Jaiku contacts are posting. Posts, RSS links and channel chatter are all available in Juhu's nice, compact UI, as are some of the nice little touches Jaiku offers, such as adding an icon to your posts and specifying your location.
All in all, Juhu is a great client for a feature-packed service that's been progressing very nicely. Fortunately, just like Jaiku, Juhu is provided free and is also open source under the MIT license. If you've been looking for a different take on the lifestream service, Juhu makes Jaiku a great option for Mac OS X users.