Well, it's taken months longer than I said it would (sorry about that). NSPSEdit 3.0 is nearly ready, and you're welcome to try it out. Features:
Completely rewritten UI
Uses the actual particle code from NS to make it as accurate as possible (many thanks to Flayra )
Allows in-map editing of particle systems (RMF only - sorry Quark users)
Implements all env_particle_custom properties (including collision detection and spawn-on-death)
Saves to a new RMF file (don't overwrite the original, just in case it all goes wrong)
Things still to be done:
Finish writing up help file with complete descriptions of system properties
Turn the "system to generate" box into a drop-down list of available systems
Fix bug where systems with gentype "Sphere" or "Blob" appear at (0,0,0) when generation entity isn't set
Fix z-fighting when "face-up" particles are created after a collision with the floor
Add language choice in settings dialog. If you want to translate NSPSEdit into another language, edit the lang/en/default.po file. Use POEdit to compile default.po or send it to me and I'll include it in the next release.
Add ability to delete existing particle systems and add new ones (for now you have to create all the env_particle_custom and generation entities in Hammer before you edit them).
This is beta software, but I'm not expecting any other bugs - if you find any, let me know (either in this thread or at email@example.com). Also, if you have any feature requests/suggestions, post them. Finally, what map forum thread would be complete without a screenshot?