Paint Shop Pro. You need 32 bit filters as the older 16 bit filters will not work. Most of the filters available are 32 bit and were created for Photoshop. Filters are readily available on the internet for free as well as commercial. Pro you will need some additional software.
Most filters come in sets and are zipped together for smaller downloads.
You will need WinZip or some other program for unzipping these files. You can get them at most shareware sites . Recently there is another method of making these downloadable files even smaller. One of the largest of all filter sites Filter Factory Gallery with largest collection of free filters is now compacting sets of filters into library files with FFL extensions before zipping them up for download. The Lite version of PICO as it is known is free.