We use Real Basic, as it is more coder-friendly as the Visual Basic by Microsoft.
We use the development environment on a Mac, but it is able to compile the code for Mac OS (PPC) and Mac OS X, Windows and Linux (x86 only, with GTK2).
Just out of interest, any particular reason why it has to be in BASIC, seeing as it's compiled anyway?
I'll second REALbasic (or REALstudio as it is now called). I also develop using it on Mac and build for Mac + Windows from the same code source. It's very nice, although it has its quirks, like everything else.