Bristol's Wind/U - Commercial product for porting Win32 and MFC applications to Linux

Abstract From Bristol's Site: Wind/U shortens your time to market by allowing you to port your Windows API and MFC-based applications, thus allowing you to develop and maintain a single high quality source code base that has native performance for Windows, UNIX, and Linux versions of your applications. Read More

WineHQ - Wine is an Open Source implementation of the Windows API on top of X and UNIX

Abstract from WineHQ site: Wine provides both a development toolkit (Winelib) for porting Windows sources to UNIX and a program loader, allowing many unmodified Windows binaries to run on x86-based UNIXes, including Linux, FreeBSD, and Solaris. Read More

The Mono Project- an open source effort to create a free implementation of the .NET development framework

Abstract from Mono Site: The Mono project is an open source effort to create a free implementation of the .NET Development Framework. Read More

REAL Software's REALBasic - commercial alternative to Microsoft Visual Basic for creating Linux apps

Abstract from REAL Software's Web Site: REALbasic is the powerful, easy to use tool for creating your own software for Windows, Macintosh and Linux. Read More

MainSoft's Visual MainWin - commercial Windows to Linux application porting platform

Abstract from MainSoft's Web Site: Visual MainWin for the UNIX and Linux platforms is an enterprise-class application-porting platform that enables software developers to develop C++ applications on Windows using Visual Studio and deploy them on UNIX and Linux operating systems.

wxWidgets gives you a single, easy-to-use API for writing GUI applications on multiple platforms

Abstract from wxWidget's Web Site: wxWidgets gives you a single, easy-to-use API for writing GUI applications on multiple platforms. Link with the appropriate library for your platform (Windows/UNIX/Mac, others coming shortly) and compiler (almost any popular C++ compiler), and your application will adopt the look and feel appropriate to that platform. On top of great GUI functionality, wxWidgets gives you: online help, network programming, streams, clipboard and drag and drop, multithreading, image loading and saving in a variety of popular formats, database support, HTML viewing and printing, and much much more. Read More