Some things I’ve been playing around with lately:
Adium – has replaced Proteus as my messaging client. I used Proteus for two years but development has slowed and Justin has moved on to work with Apple and iChat. Check out some of the Adium Xtras too (I’m using the Big Icons (Shrunk) message theme).
Smultron – a text editor that Riz pointed out. I haven’t used it that much, but it’s handy to have everything that you’re working on grouped together. It’s a pretty solid replacement for my everyday use of SubEthaEdit.
Texshop – I haven’t used it much, but I’m planning on learning the basics of TeX in the near future.
Growl – Is a universal notifier. Meaning that you can have consistent notifications from Applications like Mail and Adium.
Carbon Copy Cloner – want to make easy and complete backups? This’ll help you out.
FoxyTunes – not strictly a mac app, but pretty cool none-the-less. It’s a music controller that resides in the status bar of FireFox. Handy, considering the browser is one of my most used/open programs.