I'm a Mac user at heart. I have a couple Macs at home, but my work is a Microsoft Windows shop through and through (read: not open to non-Microsoft solutions in reality). But being that I'm actively a user in both 'realms' as well as using Linux too, I think that MacOS does have the interface that stays out of your way far beyond a Linux (Gnome or KDE) or Windows systems.
Before anyone rolls their eyes and brings in their experience with Macs from 10 years ago, or everything sucks but Windows (though they didn't like it enough to pay for it, so they pirate their Windows OS and apps), or that their relative has a Mac and it sucks (because they are treating it like Windows machine and that doesn't work)...just remember that these are my opinions, and these are what allow me to get work done without having to concern myself with maintaining a Windows machine constantly.
So, these applications listed below are how I make my life on Windows much more tolerable by saving myself tons of time and money. This list, I believe, would be useful to people regardless of their affinity for Macs. One recommendation that I'll offer before my list is that people look into and start heavily using Google's suite of online applications (Web-mail, RSS reader, office apps, mapping, calendaring, collaboration, blogging, video-chat, etc.). Honestly, if you are using Google's applications on a standards-built web-browser like Firefox...it really doesn't matter what operating system you use.
OK...I'm done with my meager soap-box...on to the list!!! This list contains traditional applications, online applications, and frameworks that tie the two together.
Drawing and Illustrating: Inkscape (Windows, Linux, MacOS)
Vector Graphics Editor, similar to Adobe Illustrator, that strives to be SVG Compliant.
Photo Touch-up and Image Manipulation: GIMP - Gnu Image Manipulation Program (Windows, Linux, MacOS)
Image edit, analogue to Adobe Photoshop.
DVD/Video Encoder: Handbrake (Windows, Linux, MacOS)
Video transcoder, and DVD ripper.
Photo Management: Picasa (Windows, Linux, and now MacOS too!)
Simple photo management tool with build in touch-up features. Similar to Apple iPhoto. Also includes a new image viewer on Windows that is FAR better than the one Windows by default uses.
Music Management: iTunes (Windows, MacOS)
In my opinion this is still the best, no-nonsense audio jukebox player available.
Audio Editing: Audacity (Windows, Linux, MacOS)
Productivity Applications and General Use:
Productivity: OpenOffice (Windows, Linux, MacOS)
Office productivity suite that is similar to Microsoft Office.
Web-Browsing: Google Chrome (Windows)
Google's offering for a web-browser. It is fast and has a nice number of clever features.
Web-Browsing #2: Mozilla Firefox (Windows, Linux, MacOS)
Mozilla's speedy and versatile web-browser.
Email-Client: Mozilla Thunderbird (Windows, Linux, MacOS)
Mozilla's e-mail client. Now...that being said...if you use a Gmail...I'm not sure there is much use for this unless you just want all your e-mail while you are offline.
Mapping/Exploring: Google Earth (Windows, Linux, MacOS)
Lets you explore anything that gets a satellite picture take of it. It also now lets you explore the skies!
Runes of Magic (Windows)
Massively Multiplayer Online (MMO) Role-playing Game that is similar to World of Warcraft in gameplay.
Silkroad Online (Windows)
MMO playing on the history of trade-routes that existed between cultures in Asia.
MMO that plays on the contrasts between light and dark forces in a medieval setting.
System Tools and Utilities:
Anti-virus: AVG Free (Windows)
Free anti-virus software for non-commercial use.
Anti-spyware: Spybot Search & Destroy (Windows)
System "Tune up": CCleaner (Windows)
Cleans up temporary files all over Windows
Disk Space: JDisk Report (Windows - uses Java)
Shows you where all the space is going on your disk.
Runtime: Java (Windows, Linux....MacOS already has it)
Allows you to run anything written in Java on your computer.
Defrag: AusLogics Disk Defrag (Windows)
A bit simpler and easier disk defragmentation tool.
Compressed Files: WinRAR (Windows, Linux, MacOS)
ZIP and RAR compressed file utility.
CD/DVD Burning: DeepBurner Free (Windows)
Free edition of their CD/DVD burning software.
PDF Printing: PDF Creator (Windows)
Allows you to print to a PDF file, instead of a printer, out of nearly any application.
Eclipse (Windows, Linux, MacOS)
Development environment that uses plugins to expand its functionality across many platforms and languages.
Combine with the Aptana Studio plugins for rich PHP, Rails, and Ajax development.
Web-based apps - available to all platforms:
Google's web-based E-mail. Includes Google Chat, along with Google's new video-chat feature, which beats Skype in quality and integrates in with all the Google features. Google Labs has plugins that also extend the features of Gmail, allowing you to put your Google Calendar or Docs right on the same interface as your e-mail.
Mapping: Google Maps
Google's maps and directions site
Productivity & Collaboration: Googe Docs
Google's online productivity suite - includes a word processor, spreadsheet, and presentation ability. Allows sharing and near-realtime collaboration.
Calendaring: Google Calendar
Probably the best calendar software on the web. Also allows sharing and publishing of calendars.
News Reader / RSS: Google Reader
A fantastic RSS newsreader or blogreader. Allows sharing of news articles.
Photo Albums: Picasa Web
Google's online photo pages. Allows tagging of people and sharing of photos.
Websites: Google Sites
Easy, template-driven website creation tool.
Google's simple and also template driven blogging tools.
Social Network: Facebook
Whoa - advocating a non-Google web-app? This is a particularly good social network.
Financial Planning: ClearCheckbook
A online money management tool.
Fuel Economy: FuelClinic
Tracks you fuel economy for multiple vehicles.
Putting together the web + the desktop:
Google Gears (Windows: IE, Firefox, Chrome ... MacOS: Firefox, Safari)
Allows Google online apps to interact with the desktop and browser, offers offline functionality to Google Docs and other applications.