One application absent from both Windows and Mac versions of Office 2010 is Visio 2010, which I occasionally use for diagramming. Though no Mac OS version of Visio exists, I could install it in Windows 7 running under Parallels if the need arose. But since I use my MacBook Air primarily as an on-the-road machine, I probably don't need to have a copy of Visio 2010 on it.
Photoshop CS 5.5
Photoshop is a key aspect of my photographic workflow--Camera Raw in particular. Photoshop on the Mac and under Windows behave quite similarly, so switching between versions is pretty simple. Also installed are a number of Photoshop plugins, including Nik Software's Dfine noise reduction and Viveza image enhancement tools, as well as Imagenomic's Portraiture.
Photoshop behaves surprisingly well on the MacBook Air. My desktop system runs Photoshop for Windows on a six-core Core i7-3830K system with 16GB of RAM, so I was a little worried that large raw files might bog down the 4GB MacBook Air; but it seems to work fine, as long as I limit myself to keeping one or two images open. The noise reduction plugin runs noticeably slower under these conditions, however.
I often need to capture screenshots of applications, desktop shots, and related items. Snagit Pro, which comes in both Mac and PC versions, is my application of choice. The two behave slightly differently, but the user interface is similar enough that I'm comfortable switching between versions.
I use a few additional tools to keep data in sync between the MacBook Air and my Windows systems.
I'm sure Safari is a capable enough browser, but with Chrome's cloud-based bookmark sync feature I can easily keep the same set of bookmarks across all versions. I've noticed that Adobe Flash seems to impose a bigger performance hit and to be less stable on Mac OS than on Windows. But then, Flash isn't a model of stability on Windows, either.
Dropbox (and SkyDrive)
I've used Dropbox for several years now, and its capabilities have grown significantly in that time. I currently have Dropbox on my Windows system, on Mac OS, and on my iPad and iPhone. It's been incredibly useful, enabling me to share large files with editors when email won't work, and to move artwork or text files between various systems.
I've just started playing with the latest iteration of Microsoft's SkyDrive. I now have it installed on the MacBook Air, and with the free 25GB available for my use--one of the benefits of my being an early adopter--it makes moving large photographs and other data files around even easier.
When working with either storage service, I've used the synchronized folders as my default save folders for all my Word and Excel documents. That way, they instantly show up on my Windows-based home systems.
For quick and dirty notes and lists, I use Evernote. It runs across Mac OS, Android (which I don't use), Windows, and iOS.
Windows Home Server Integration