Depends on what you are looking for, how large your development team is, what size the project is going to be, and what your main objectives are.
If main priority is being cross platform with the least developer effort, have a look at Java. It's cross platform right from the start, and it's a very productive language and allows a relatively small team to manage a fairly large project (all subjective, of course
) without the need for very specialized knowledge. The downsides are the GUI fidelity will be highly suspect on all platforms. Your application will require more memory, and the GUI will look out of place on all platforms. Swing can emulate the native look and feel, and while it does quite an admirable job with the look, the feel leaves something to be desired.
If you're keen on performance and GUI fidelity, use C++ with one of the cross platform application frameworks. My vote will go to wxWidgets, since it's quite similar to MFC (for better or for worse...) thus many programmers Windows
will be familiar with it. The widgets are native, so you're app will look quite good on all platforms, and the license is LGPL, which means you can distribute your applications without revealing the source provided you dynamically link to the wxWidgets library. There is of course Qt, which is another popular framework, but you I prefer wxWidgets because Qt's GUI components are all emulated (a la Java Swing) so the look isn't always correct, the license is either GPL (reveal all your application's source code) or commercial ($1500 per license per platform). The benefits to Qt is that it is coded in much cleaner C++, compared to wxWidgets convoluted message map, which is implemented by way of C++ macros. My advice is to have a look at these toolkits, and decide which one suits your needs.
If you're looking for RAD, and quick time to market, you may want to look at REALBasic as it is quite similar to VB and the executables run on Linux, OS X and Windows. The downsides I can see are that it is quite expensive for a small shop, and it uses the point and click interface which makes knocking up an application easy, but maintaining that said app a nightmare, especially if you have to maintain someone else's code. Another RAD option could be Python, with bindings for one of the cross platform GUI frameworks. There is wxPython and PyQt but I've never used Python so I can't say anything here.
Hope that helps you.