It would be nice if GNUstep (http://www.gnustep.org/) received much more attention. GNUstep is a free software implementation of the OpenStep API (which Apple's Cocoa API is based on), and GNUstep is available for *nix and Windows. The OpenStep API has already been shown to be cross platform; back in the days when NeXT, Inc. existed, it sold an implementation of the OpenStep API and development tools for Windows NT called OPENSTEP Enterprise (see http://i.imgur.com/I7VivtO.jpg for a screenshot). After Apple bought NeXT, Apple renamed OPENSTEP Enterprise "Yellow Box for Windows" during their Rhapsody efforts. However, in 1998 Apple's business strategy shifted from maintaining NeXT's cross-platform support for OpenStep to devoting NeXT technology solely to the Mac. This is what killed OpenStep as a cross-platform development technology.

GNUstep could have picked up from where Apple left off in 1998 to be a major contender in cross-platform software development. Indeed; it could have attracted former OpenStep developers who liked the cross-platform nature of the API and were stranded when Apple stopped development of Yellow Box for Windows. But for various reasons GNUstep's development and adoption has been slow, and alternative APIs such as QT and GTK ended up winning out in terms of popularity. But I don't think it's too late to promote GNUstep, especially now when a vocal minority of Mac power users are increasingly becoming discontented with the state of the Mac and are looking for high-quality alternatives.