I’ve been running a Mac as my work machine for a while now. (And, since it’s a laptop, it tends to become my personal machine somewhat, too.) Excluding things that are specific to programming, here are a few tools I don’t think I could live without–and they’re all free!

• smcFanControl. (Wow, my version is way out of date.) My machine can hit 75 degrees Celsius, at which point parts of the laptop are too hot to touch, and things inside can’t be happy. Apple seems to have designed the machine to stay quiet: even as I’m at 75 degrees, the fans don’t ever exceed 1,000rpm. But they go to 6,000 rpm, and smcFanControl will take you there. It’s not until somewhere around 3,000rpm that I even start to hear the fan, and it makes a big difference. (6,000 rpm can bring the CPU and chassis down 20 degrees Celsius in under a minute.)
• iStat Menus, which place helpful (and customizable) things like network speed, memory utilization, and CPU graphs in the Apple bar (whatever it’s called: the menu bar across the top of the screen). With 2GB RAM (and a tendancy to have way too much stuff open), it’s handy to keep an eye on its usage, and it takes maybe 20×20 pixels to do so. Clicking on the graph brings up a spiffy graph and a breakdown of the top processes using RAM.
• QuickSilver, and I don’t even use it for half its potential. Ctrl+Space and start typing, and it’ll match things. “fir[Enter]” and I’m in Firefox, “ter[Enter]” and I’m in the terminal, “tex[Enter]” and I’m in TextMate… It’s actually capable of much more, too.
• Firefox and Thunderbird. Even though the Mac comes with Safari and a pretty slick mail client, I think power users will feel most comfortable with these. And both are quite customized (and plugin-ized) just to my liking.