All good suggestions here, but I’ll put in a word not to use an antivirus. It is a waste of RAM and processor cycles, hence battery life as well, and it won’t provide any benefits whatsoever. Heck if anything, antivirus software on a Mac probably ends up adding additional vulnerabilities on balance.
Anyway, since it sounds like you are new to OS X, as you get familiar with it you’ll probably find a number of things that you wish you could customize. To that end, I’ll recommend:
TinkerTool to access hidden settings
BetterTouchTool to create custom gestures and shortcuts, as well as provide window-snapping
For office software, obviously Microsoft makes the go-to standard, and LibreOffice is quite good. Right now Apple’s iWork programs are kind of nerfed, though they are slowly adding back the functionality they lost last year.
If you want a small, light, streamlined word processor, it’s hard to go wrong with Bean. I wish it could do footnotes, but for day-to-day document-writing it’s capable and snappy.
TextWrangler, as mentioned, is quite powerful.
I second the Seashore / Pixelmator options for image editing. Also iDraw or Sketch if you work with vector art.
If you do any audio work, Audicity is free.
Boom lets you increase your speaker volume, set up a system-wide equalizer, and boost the volume of audio files.
The Unarchiver lets you expand a plethora of file formats.
If you’re getting a retina machine, QuickRes lets you adjust the resolution quickly from the menubar, as well as use true native resolution if you want.
Let’s see…for torrents there is Transmission among other programs. For charts, GraphSketcher just went open source. For other stuff, it depends on your needs.
I’ve got FreeMat (a Matlab clone), Ukelele (a keyboard layout editor), KeyRemap4MacBook (for serious keyboard modding—like making shift keys type parens when tapped quickly), PCKeyboardHack (for low-level keyboard modding to allow remapping of keys to functions that are already remapped—like making the control key act as Cmd-Opt-Ctrl when the caps lock key is set to act as Ctrl). Hmm, I guess I take my keyboard setup seriously.
