After attending the 2005 Apple Developers' Conference with a borrowed IBook, I started seriously considering an Apple "Switch" for my next computer purchase. Linux support for laptops has come a long way, but it is still an uphill battle to get things running as smoothly as they need to under the demands of a mobile environment. I have grown weary of fighting that battle every time I upgrade distros or laptop hardware.
When the new MacBook Pros were announced, I held back because of their size. Having used a Dell CS lattitude (14"), an Acer Travelmate (15" widescreen) as well as my wife's Powerbook 15", I had settled on wanting a nice small laptop that would be easy to carry around. When the MacBooks were announced, I jumped to order one.
Finding all the apps I use on a daily basis was pretty easy, because most of those were built in. Apple's Mail utility is very nice, but due to some missing functionality, I've been using Mozilla Thunderbird instead. Mozilla Firefox was another first day download.
XCode comes with the OS X DVD, but is not installed by default, so I typically include that so I'll have all my usual development environment tools, gcc included. It's also necessary for X11 support in OS X.
As for office applications, I include MS Office X just because I can get it for cheap from work, but I hardly ever use it. For my own purposes, I prefer Open Office. Here, I reached a dilemma. There are two versions of Open Office available at present for the Mac, and neither is ideal. One, NeoOffice is compliant with the Mac Aqua environment, but in my experience with it was rather slow and lagged too far behind the latest release version of Open Office for other platforms. The version I prefer now is the more up to date Open Office version but it requires X11 to run and is not at all integrated into the OS X aqua/cocoa/carbon interface. This means you have to remember a different set of keyboard shortcuts, cannot drag and drop, print via bonjour, etc.
Pulling my favorite software from Linux/Windows only goes so far. With a little help from friends, I've been filling out my list of mac only software as well.
Two of the first great finds were Adium X for IM and Desktop Manager to give me multiple desktops a la Linux
More recently, I was told about Colloquy, a great IRC client, and Quicksilver which is a really efficient cross between a launcher like the dock, a search widget like spotlight, and a raw command shell. (Thanks Venkat!)
Most of what I've mentioned so far are basic needs for most users of a computer, web, mail, word processing, IM, etc. Building from that are a few key applications I've found useful in my trade, systems administration.
As a Unix systems admin, I tend to spend a lot of time at the command line. Apple's built-in terminal works just fine for me. It has great support for different terminal environments and characters, and with the base line of a BSD system, I have all the tools I'm used to.
One application I like to add for command line work is SSHKeychain. This handy app keeps track of your ssh keys and ensures that any other app that needs access to those keys, such as ssh or Fugu (GUI SFTP client) gets it with integration with the OS X Keychain. This means you can actually generate separate key pairs for each account, assign them all different passwords (you do password protect your ssh keys, don't you?) and keep track of them with your login password only.
Finally, for keeping track of all those passwords safely, I found KeePassX
CoolMacApps is an interesting site that aims to obsolete the ever popular top-ten cool apps lists. It's by no means complete, but since it is supposed to be community-maintained by votes and submissions, it has promise. Already, it's helped me to replace DesktopManager with Virtue Desktops on my own list, since Desktop Manager doesn't have support past Panther.