I'm testing the beta of Mixero as I type this, and this app really delivers on it's promise of "reducing the noise." The interface is really nicely done and there are only a few things that a little less than intuitive but fairly easy to figure out as you use it. It's got a really nice feature set [see the list on http://mixero.com] that helps make your time spent on Twitter much more productive. Follow them at => http://twitter.com/mixero to get the scoop on their latest activities (hint: Facebook integration sounds cool) and bugfixes.
It feels different than other Twitter clients I've used -- feels more streamlined and the interaction is more directed towards productivity as opposed to "stream following" like other clients. I'm still not a huge fan of apps built on Adobe Air because this machine is old and just doesn't have the horsepower necessary to really handle "power-browsing" while running an Air app and a handful of others simultaneously (you can never have enough RAM). Because it's an Air app, it should work on any OS, and as I mentioned the look and feel is definitely silky.
As social media continues to converge and inter-operate -- it's getting increasingly more difficult to stay on top of it all. Although I've only been testing it a short while, I can see that it's definitely going to be an important tool in my social media tool chest.