Software written for Mac that was later ported to Windows What notable software is there that was originally written for Macintosh and then later ported to Windows? What makes this software notable?

Let's stick to actual software packages that stand on their own and not ideas or implementations like a window system or menu idiom.
Microsoft Word in 1984 
Microsoft Excel in 1984 
Aldus PageMaker (later Adobe PageMaker), circa 1986
Adobe Illustrator, 1986
Fontographer, the first commercial scalable-font authoring software, 1986.
Microsoft PowerPoint, 1987 (originally called Forethought Presenter) 
Adobe Photoshop circa 1987 
Adobe Illustrator circa 1987
QuarkXpress, 1987
Macromedia Freehand
Avid, the first professional personal computer-based video editing software, 1988
Make Music Finale circa 1989, the publishing industry's leading application for typesetting sheet music
AOL, 1990-1991. America Online and its client software application were originally Mac-only.
Digidesign Sound Tools and Pro Tools, starting 1989
Adobe Type Manager, circa 1990. The first scalable screen fonts on a computer display.
FileMaker Pro database software (an independent division of Apple), circa 1986
Apple QuickTime
Apple iTunes
Apple Safari
Scrivener, 2009


The Mac and its operating system went on the market in 1984. Remember that while Microsoft Windows existed in various forms in the late 1980s, it did not catch on and become widely adopted until Windows 3.1 in 1991. Before that time virtually all PC users were still using MS-DOS with a text-only interface and no graphics, GUI or mouse.

Microsoft provided text-only applications called Word and Excel (no graphical user interface or mouse) for MS-DOS before 1985, but the first programs called Word and Excel with a GUI were designed and written for the Macintosh. It's not accurate to call them a "port" from MS-DOS as they had to be re-designed and implemented with the Apple Macintosh assembly language and proprietary new API for the mouse, GUI and specific user interface elements in mind. So practically speaking, Word and Excel as we know them were first written for Mac and then ported to Windows.

The entire industry of digital photo editing and desktop publishing was born on the Macintosh platform, which was the first platform to support PostScript laser printers (and Apple designed and marketed the first of those printers, the Apple LaserWriters, with PostScript software licensed from Adobe. HP or Hewlett-Packard laser printers did not support PostScript, resolution-independent fonts, or graphics until some time later.)

All of the original software applications for editing photographs and doing text layout and typography first appeared on the Mac. Those that survived the marketplace competition and flourished were later ported to Windows. This includes essentially the entire Adobe product portfolio throughout the 1980s.

Microsoft and Adobe also bought software from smaller independent companies and adapted it to their needs. The predecessor to PowerPoint was an independent Mac-only company, and the very first version of Microsoft PowerPoint was Mac-only. Aldus was an independent company that made the first great desktop publishing program, PageMaker. It was bought by Adobe.

The adoption of personal computers for editing film and video for the industries of motion pictures and broadcast television also began with applications on the Macintosh platform, starting with the Avid offline film editing program. 

The same thing happened with the music and recording industry. While there were MIDI sequencer applications for MS-DOS and other platforms, the first widely-successful personal-computer-based programs for recording and editing music appeared on the Mac, from Digidesign and others. This hastened the demise of professional multi-track analog tape recorders. I should mention that with regard to music and audio, capable Windows-based programs that were not ports (Cubase, Sony Acid, etc.) appeared very shortly after the first Mac programs. At this point, in the early-to-mid 1990s, computer processing power was more up to the task and Windows was becoming a viable alternative to the Mac platform, so implementing solutions on Windows rather than the Mac became more attractive to developers.

AOL (America Online) was originally for Mac users only and its original software was licensed from Apple Computer. But some time after the launch for Mac users, AOL ported their GUI Mac application to Windows and accepted Windows users. All this was well before AOL had any connection to the Internet. It was a stand-alone closed system accessible directly by dial-up modems.

Before Adobe Type Manager, circa 1990, computers could not display fonts at any arbitrary point size, or "zoom in". Fonts for screen display before that time were only fixed-size bitmaps, and text on the screen of Macs or Windows could look quite blocky and hard to read. I seem to remember that Adobe Type Manager for Mac predated Adobe Type Manager for Windows by some length of time, but I have not confirmed this. Subsequently Apple and Microsoft worked jointly to develop the TrueType font scaling specification and method of screen display, which was rolled into the Mac operating system and Windows, circa Windows 98 in 1998. This made Adobe Type Manager obsolete. I have not researched the exact dates.

The entire Microsoft software development division that wrote games for the XBox was created when Microsoft bought Bungee, which was up until that time a Mac-only game company. When Microsoft absorbed Bungee, they cancelled all their Mac products. The developers who created Halo, therefore, had been working on Mac products only--up until the time that Halo for XBox was released.