Used to be that Windows users could admit the Mac was easy to use--they just complained there wasn't any software for the platform. Apple knew their weakness, too, so they endeavored to turn their developer tools into the envy of the industry. What they've created in the latest versions of Xcode and Interface Builder, two of the anchor apps in the Mac developer's kit, are an engineer's dream team. Their thoughtful, intuitive design beget tools for the Macintosh that are just as much about visual design--gorgeous graphics and standardized controls--as they are about ingenious, robust interactivity.
So it's no surprise that in the nearly nine years that Mac OS X has been earning converts, -the Macintosh a decade after the first iMac has become a software honeypot luring Windows users from all walks of life. While some of the apps below have counterparts in the PC realm that aim to do a similar task, the apps on this list say as much about the Apple-inspired philosophy of interaction--elegant, efficient, easy, powerful--as they do about the ideas that drive Mac developers. Below, the apps that Windows users can only wish came in .exe.
Tangerine! is the long lost companion to iTunes: it lets you create playlists by the beat and intensity of your music, allowing you to create purpose built lists for relaxation, exercise, parties, and work without having to slog through the tens of thousands of songs in your library. Very much in Apple form, Tangerine! has a three-pane browser that automatically culls your iTunes music and analyzes it for its musical properties, then plugs your playlists back into iTunes once you've made them. $25.
If you're obsessive about your media collection, Delicious Library 2 is a must-have. Hold any book, CD, DVD or video game up to the iSight camera on your Mac, and DL2 reads the barcode, storing that piece of media in your library along with all the data that goes with it: reviews, summaries, links to buy and sell on Amazon, synopses, and suggestions for similar media. (You can put in anything without a barcode by hand.) Not only is this good for remembering who's borrowed what, or providing evidence in case of an insurance claim--it also makes building bibliographies, organizing your stuff, and doing research with your own library a lot easier. The best part: you can publish your library to MobileMe, allowing you and your friends to share libraries, so you can pool your resources. $40.
Think of Acorn as everything you need from Photoshop, and nothing you don't. For essential, light image editing, Acorn is incredibly fast, versatile, customizable and cheap at just $50. Made with care by a well-respected Seattle developer, Acorn could teach the engineers at Adobe a thing or two about interface design.
Talk to any hardcore Mac nerd, and there's no doubt he'll admit to living and dying by Quicksilver. It's a launcher, at heart--a few quick key taps and you can fire up any app on your machine. But it's also ingeniously customizable. Hit the hotkey and punch in anything else: Filenames, contacts, URLs, iTunes commands ("pause"), and complex daisy chains of actions, and Quicksilver fires them off with alacrity. And it's free. (Thanks to Quicksilver, this writer goes hours without touching a mouse.)
Spaces by Apple 
Virtual screen space isn't an idea Apple owns, but it's certainly one they've perfected. Spaces allows you to create virtual desktops and assign which apps live in which space. While the idea sounds great in concept, it'd be easy to royally botch in practice--but because of Apple's superb graphical transitions, smart commands and hotkeys, and ingenious logic, Spaces gives you more real estate, and more control over it, than a whole desk full of monitors. (Comes with Mac OS X Leopard.)
Fluid App 
Fluid is a free app that allows you to create freestanding applications from any of your favorite websites. It's essentially a single-purpose browser, but allows you to assign certain sites you use a lot (like say, a CMS) to certain Spaces in OS X, and even makes them run faster. It's also light, quick-building and super-stable. (Below, a Reddit app I created for that site's technology channel.)
If you've ever used an invoicing app for Mac or Windows, you know that they are largely nightmarishly complex. Billings has such an elegant workflow and a carefully-considered user interface that it snagged Apple's highest honor at WWDC: an Apple Design Award. And in true Mac tradition, it integrates fully with Address Book, iCal and Google Maps all within the app. A companion iPhone app is on the way. $40.
LiveView by Ideo 
LiveView is actually two apps: One for iPhone and one for the Mac. Fire it up on the Mac, and you get a big, glossy iPhone bezel on your screen. Anything that's within the bezel gets beamed to any iPhones on your LAN that are running the companion app, allowing all your buddies to pipe in on your design or programming work whenever they like. Free.
Aperture by Apple 
For photographers, there simply isn't anything remotely as robust for photo management. What about Adobe Photoshop and Lightroom, you say? Well, that's a great combo for heavy editing--but when you have a library of 50,000 RAW-format photos to manage and circulate, Aperture is the clear winner. Its dead-simple organizational system makes it easy to sort and stack, and because of its robust backend, it can handle and back up photo collections that number in the hundreds of gigabytes. Got MobileMe? It can auto-publish certain albums to the Web, too. Bonus feature: you can order professional quality prints up to 20 inches by 30 inches for a fraction of what a print shop would charge. $200.
Blogging used to be a light, simple endeavor, but with the advent of Tumblr, embedded media, and reblogging, these days it takes a desktop app to do it right. MarsEdit 2 lets you build posts, preview, publish and edit without touching a browser, speeding up what can otherwise be a painful Web-based chore. It feels a lot like Apple Mail, to its credit, and lets you set up custom macros so you're not constantly re-typing the same stuff. $30.