PSDeveloper BETA is a Windows application that will allow you to develop for the PSP. This homebrew application works on both Windows Vista and Windows XP. Here's a list of features of TRStealthX' PSDeveloper BETA has:

Insert common, media, and net headers.
Insert callback routines.
Example codes of Hello World and Displaying an image to the PSP.
Allows for basic file operations

What to expect from the app in the future according to the homebrew dev:

Syntax highliter for better, and easier coding
Makefile maker
Auto define function (defines everything in your code, so you can understand what your doing better)
SmartBuild funcion - allows for compiling and testing straight off PSDeveloper
More code examples
Way better GUI
Less bugs
More, way more