The first and foremost alternative to REALbasic is FutureBASIC from Staz Software. Although not cross-platform, its support of the Macintosh is very strong. Not only does it fully support both native Mac OS X and Classic PowerPC compilations, FutureBASIC continues support for 68K compilations, the only commercial compiler that I know of which still does. Moreover, it even supports building apps that will run on pre-System 7 machines! The same project can be used to build an app to run on a Mac Plus running System 6.0.5 or as one to run on a Power Mac G5 running Mac OS X 10.4 Tiger. That's dedication, man.

While REALbasic enforces an object-oriented coding paradigm, FutureBASIC allows for more structured programming, a simpler and more common approach for most Basic programmers. For those who need to be jump started, FB also includes a rather large number of sample projects to start you off with.