I forgot to mention that I’m participating, so might as well before the event starts!

Tools I’ll be using:

https://app.hacknplan.com (planning, saw an endorsement for the program sometime this morning, if it’s one of the themes that I had sudden inspiration for, I’ll have plans)
Gimp and/or Inkscape (graphics)

Unity3D (yes, I have given in and decided to use the horrible, imfamous, cheaty-mc-cheat-cheat dev tool, Unity3D)

BFXR (sound effects)
OpenMPT and Audacity (music)