01- Is there any better C++ tool (IDE) other than NetBeans and MS Visual Studio? Better means very mature and popular (and free).
That's going to end in a flame war. I like emacs. It's now been more than 20 years old (very mature!) and quite popular. eclipse with CDT is also very popular and has been around for serveral years now. Choose the one you like the best.
02- What is the difference between GNU Java and Sun Java compilers?
The GNU Java compiler is just gcc with a java frontend. It can for example compile your java code into native binary code. The Sun java compiler is from Sun and is more mature and it's regarded as the reference implementation of java compilers.
03- Is there any other popular C/ObjC/C++ complier out which is free, mature and popular?
If by free you mean free as in free speech (open source code available), then look for llvm which can compile C++/C/Obj-C code (using gcc as a frontend. llvm is a compiler infrastructure. It's not got the actual code). Douglas Gregor works on a native llvm frontend for C++, without the need of gcc: http://clang.llvm.org/ . LCC is small C compiler you can use too on Unix. I've read there is also a Win32 version.
If by free you mean free as in beer, you can of course also take the Microsoft C++ compiler (shipped with Visual C++ Express) cl , but it will only work on Windows.