I have an old version, 5 I think, of psp, but I never use it anymore . I have ps elements4, ps7 and cs2 all installed. I cant think of anything I want to do that can't be done with these programs.
I use PhotoShop but I also use Paint Shop Pro Version 10. It is a very good program and very PhotoShop-like in many of its features and how you use them. Perhaps the difference is that PaintShop Pro is very fast to work with and produces excellent results that are comparable with PhotoShop. I am now finding that the available Plug-ins that work with both PhotoShop and PaintShop Pro are more important than some of the features of the programs themselves.
It is great for example to selectively use a polarizer, graduated neutral density filter, skylight filter, classical soft focus filter, etc. in postprocessing after the photo has already been shot with minimal loss in quality in 16 bit mode.
I use photoshop. I have not used the recent versions of PSP so I can't help you out there. The GIMP is quite good and is freeware but is not as easy to use as photoshop. There was a release of GIMP called GIMPshop which has a GUI that is very similar to photoshop and is also free