Gambas is a free development environment based on a Basic interpreter
with object extensions, like Visual Basic(tm) (but it is NOT a clone!).
With Gambas, you can quickly design your program GUI, access MySQL or
PostgreSQL databases, control KDE applications with DCOP, translate
your program into many languages, create network applications easily,
build RPMs of your apps automatically, and so on... You have to install
the necessary components

Gambas is still in development, but already useful for smaller projects.
The IDE itself is written in Gambas-BASIC. The source can be used reviewed
as an example application in the /usr/share/gambas/examples directory.
Documentation is also in that directory.