Sparrow - Super simple email client, the closest (and best) thing you can get to native Gmail on the Mac. It’s got a one up on Mail.app as well since it actually has a menubar icon.
Adium - MSN is still my protocol of choice and since Microsoft Messenger is horrible on the Mac I use Adium, fitted out with a nice iChat style menubar icon which you can find over at adiumxtras.com.
Tweetie - I’m still stuck on PowerPC, so Tweetie is my best option. Pity me. It may not be the best, but it’s still a solid choice even now.
Dropbox - Self explanatory super sync’y goodness.
Keyboard Maestro - A powerful macro program I’m playing with before I bite the bullet and buy it. I use an amazing alternative menubar icon made by Jono Hunt over at Iconaholic.
Pastebot Sync - More super sync’y goodness. Lets me get copied items to and from my iPod Touch and my Mac easily.
Built in stuff - LsSaAlerter (I have no idea how to get rid of it), AirPort, Volume, Time and Spotlight (I can’t get rid of it either so it just sits there, Alfred is my choice of launcher instead).
(From Huw Martin)