Came across a few more last night!

Audioboost v0.0.2b
This plugin raises the volume limit of your PSP's audio output on both speakers and headphones.
(I personally tried this myself and it definitely increased the sound output through my PSP's speakers and earbuds and not to a point where everything sounds distorted)

SimplySaver
This plugin allows you to save files you download from the PSP's internet browser anywhere on your Memory Stick.
(For example: You can save an MP3 into the picture folder, an MP4 movie into the music folder, and save other files anywhere)

ResourceHidder
Similar to what Preview Contents Hidder does, but with more options such as skip the gameboot & the PSP logo on the gameboot.

By the way, Gazup isn't working for me at this moment in both Firefox and IE, it's giving me an error about URI disallowed or something