There are no tools for performance tunning. I haven't tried Xab yet. From what I have seen in the literature it is a good debugger. For performance tunning, I have been creating my own trace files in PICL format. It helped enormously.
Paragraph is an excellent tool.
PVM, PICL and Paragraph are public domain tools. Such a standard will be extremely useful. If you are really lucky, the C compiler has no bugs, and on miraculous systems the operating system is pretty stable too. Here, when I say C, I mean C and probably Fortran too. My best experience has been on the the Cray machines.