If you’re developing software that will run on multiple platforms, in most cases the technology you’re using will handle rendering controls and widgets in a platform-appropriate way. But using platform-specific design patterns is still up to you. Luckily, most major platforms provide guides for designing UIs that feel right. Here is a list of useful documentation broken out by OS.

Windows

• Windows User Experience Interaction Guidelines include sections on controls, windows, commands (by which they mean menus and toolbars), text, handling input devices, and more.
• The Aesthetics section of the above document, in particular, has some really useful information about UI layout and the graphic design side of UI.
• Microsoft’s overview of the Windows Environment will be particularly helpful for anyone who hasn’t been able to keep up with the Windows UI over the past few revisions.
• MS’s Visual Index of controls provides screenshots and brief descriptions of UI controls as they appear in the modern Aero Windows graphical theme.
• Top Guidelines Violations serves as an example of what not to do in your UI design.
• How to Design a Great User Experience offers 19 pieces of advice. I’m not certain that Microsoft is the best source for advice on creating great UIs, but they have been successful, and most of these points are fairly self-evident.

Mac OS X

• The Apple Human Interface Guidelines are the grandfather of all UI guidelines and pretty much a one-stop-shop for information of best practices in Mac OS UI development.
• The Aqua Interface deals specifically with the controls and widgets available to UI developers on the Mac.
• The Mac OS X Environment provides a run-down on the Dock, Finder, Desktop, and more Mac-specific OS-level features that may be unfamiliar to Windows-only developers or designers.
• The Design Process is a good, quick read with some good, though often obvious, advice.

Linux/Other

• The GNOME Human Interface Guidelines are similar in layout and organization to MS and Apple’s guides, but cover the GNOME desktop environment.
• The KDE User Interface Guidelines provide the same for the KDE environment.

This is not intended to be an exhaustive list of UI reference materials; rather, it is a list of official reference materials provided by platform providers. If you know of other useful platform-specific UI guideline documents, please post them in the comments.