Create a cross platform Windows, Mac OS X application

It's good that you're thinking of portability early on - it's vastly more difficult to "bolt it on" after the fact.

There are various cross-platform kits available, but IMHO all of them fall a bit short of providing a "native" look and feel on all the supported platforms. On the Mac (what I use), proponents of such kits always want to mention that they're using native controls. That's a good start, but it's not the whole journey. Other issues addressed by Apple's Human Interface Guidelines include how the controls should be arranged, how button labels should be phrased, what standard shortcut keys should be used, etc.

Even Microsoft had to learn the hard way about the dangers of trying to write a cross-platform GUI, with the ill-fated Word 6.0 for Mac.

IMHO, a better approach is to use an MVC design, with the model layer written in standard, portable C++, and the view and controller layers using the native toolkit for each platform. For the Mac version, Carbon and C++ throughout used to be an interesting option that is now not supported anymore, so you would want to use Cocoa, using Objective-C in the view and Objective-C++ in your controllers to bridge the language gap. Your Windows version could likewise compile your model as "managed C++", and use any .NET language for controllers and views.

Take a look at Real Studio. Seriously. You can write an app in Real Studio and deploy it on Windows, Mac OS X, and Linux.

Edit: Real Studio is now Xojo.

wxWidgets is a cross platform C++ library, which is a practical choice. But I agree with Sherm - all cross platform libraries do create an inferior UI to native applications.

It's made harder by each OS having different UI semantics (button orders etc), so while you may achieve a good look, getting the 'feel' right on each platform via one view layer is almost bound to be impossible.

Depending on what you end up doing, you may find a web interface better (e.g. embed a web server in your app and serve HTTP pages to a browser). You avoid the L&F issues then!

Alternatively, you can decide you're just going to have a completely non-standard L&F, and go for something like wxWidgets, or Tcl/Tk.