i work primarily in Paint Shop Pro. i've used the gimp and gimpshop and some others, but i just prefer psp. i've had it since version 4.12 (which was free back then).
i use a number of the same filters and plugins that others have mentioned, but my current passion is Filter Forge. This is filter a
maker and not just a filter itself. there are currently over 1500 filters in the library and it's still in beta! this is one amazing program. you dont need to know any code. it's plug 'n play type filter making, all graphical. you just plug one thing into another and poof!, you've got a filter. plug some controls into those things and you've got a filter you can adjust. quite simple, but surprisingly powerful.
my most recent filter that i made was an attempt to make a paint program. now, how's that for a twist, a filter that makes a paint program

i can sharpen, blur, black and white adjust, noise addition, noise removal, HDRI lighting, kaleidoscope effects, offsets, simple masking, tone changes, simple channel mixing (in the latest, un-released version), perlin noise effects, threshold effects, and more.
the program was originally developed to make textures. but since its inception it's grown way above that. there are several folks in there (myself included) making artifying type filters and some are really quite good.
oh, and filter forge can work as both a plugin and a stand-alone.
i do mostly restorations and photo art. for the restorations i use a number of the other programs mentioned already. and for photo art, i've been using mostly Paint Shop Pro and filter forge. the two in combination make a great duo.