As much as I love Linux, I actually do all of my work from a Powerbook (1.5GHz, 512MB RAM, OS X 10.4), and I have to say that it is great! This is the first that I have used a Mac since my old II cx with OS 6.x (or so). OS X interacts wonderfully with all the stuff I do on Linux - with X11, I am able to open up X applications remotely and have them display on my Mac. It also works well with Windows, mounting network drives, sharing printers, and all that sort of thing.
Here are just a few applications I find myself using on the Mac.
Fink can be found at http://fink.sourceforge.net - It is a project which has ported a lot of useful open source UNIX software over to OS X. You can use apt-get and similar to install binary packages, or you can download and build from source. This is great because it lets you use pretty much anything that you would want to use on *NIX.
iTerm is a very useful and easy to use terminal replacment. Now I can use the middle mouse button for pasting (like UNIX), and it also has tabs - yay! I spend most of my time in terminals, and so iTerm is probably the application I use most.
Curious about what wireless networks are floating around? Download MacStumbler, and you can see what access points are nearby, with signal strengths for each.
Another useful program for testing wireless networks is KisMAC. Currently KisMAC has been updated much more recently than MacStumbler.
This is a timer application which has all sorts of features I don't use, but is useful for a countdown timer, like for making tea. I use it with the 'Glass' sound to give a gong-like reminder at the end of meditation sessions. Web site is here.
Of course, many of the programs I use come with the OS. iChat lets you talk to AIM users, as well as people on your LAN. iPhoto is great for all your digital pics. Disk Utility allows you to make .dmg (disk image) files, as well as burn CD/DVD .iso images. You can also burn CDs and DVDs from the Finder, or from iTunes, iPhoto, etc - everything is very well integrated. Ok, enough ranting about how much I love the Mac... :)