If you haven't acquainted yourself with Twitter yet, its a social networking and micro-blogging service that allows its users to send and read other users' updates (known as tweets). It's a bit like the status update feature on Facebook but on steroids.
Scalaris is a Twitter client that's designed to be as discreet and minimal as possible. It shows both your contact's tweets and like any good client should, supports Growl notifications. Scalaris sits quietly in your menubar so it doesn't take up any Dock estate like some clients do and is set by default to retrieve tweets every 90 seconds which you'll receive as Growl alerts. There seems no way to increase this frequency which is strange although 90 seconds is probably enough for most people's needs.
The good thing about using Growl is that Scalaris gives you a lot of control over how your tweets and received. You can change Growl notification settings in the System Preferences which will immediately be applied to Scalaris tweets. To send your own tweets, you have to bring up the Scalaris window which is perhaps the only slightly annoying thing. To do this, press CTRL+ the up arrow on your keyboard to reveal it. It would be preferable to have this accessible by mouse or visible permanently although that would make Scalaris less discreet which is one of it's strengths.
Scalaris is a super discreet and lightweight twitter client that makes Growl do all the hard work so that your tweeting doesn't get in the way of your workflow.