Here's a list of my top picks, (all of which can be found on Qj) just search for it under PSP.
1). PSP Filer 3.1- This app is great, if you like to experiment, or save flash files. It even makes you enter " mode" before you can flash anything.
2). Y-Paint 5.00 beta- This is probably the best and easiest to use paint app around.
3). Easy PSP Calculator- Nuff said. Easy to use and will basically cover all MS/HS math problems.