Real Basic is based on the "basic" syntax but it isn't really anything like VB6. Visual Basic 6 is a component orientated language and hence is platform specific to the components on which it depends. This method of programming is within the COM+ model and is exclusive to Microsoft.
MONO .Net by Novel is the closest thing you will get really but your talking about .Net languages where the syntax is similar enough in fact identical and extended and lacking in COM+ dependencies.
VB .Net is a worthwhile upgrade as its not only free but introduces a new level of sophistication and power. Being able to create VB modules in the VB6 world is also possible and then you could import the DLL or library that you created with VB6 and then import it into your includes within a .Net language such as VB .Net, C# .Net or even C++ .Net and compile it within the Mono environment on Linux.
http://www.mono-project.com
There are two branches of the open source .Net IDE SharpDevelop and as one of them came to an end it's now MonoDevelop for Linux exclusivity. This is based on Mono and if nothing more, quite a good place to start having a look at upgrading your skills from VB6 world as man, being honest thats old hat and .Net blows it's usefulness to pieces, except for that single fact - you know how to program already so it's not like your having to reinvent the wheel.
As for a commercially viable and quite matured IDE which is big with Java people, lends itself to .Net and kicks serious butt - Eclipse is your friend - really a great development environment and so portable that once you are setup and working with it, adding new languages is like child's play.
http://www.eclipse.org/
Bingo! You have a complete open source development environment which is able to do far more than you ever could with Visual Basic 6. Go write some code!