Though I don't mod, I do program using GML (Game Maker Language). Whenever I work on my Cave Story project, I listen to music. This includes conceptual art, graphics, 3D modeling, and programming. I will rarely listen to music if I'm writing story or brainstorming on game mechanics and such though. Also, I will never listen to music if I am making music for a game... haha, sorry, I thought it was a little funny...
The music I listen to will usually be from a video game. Recently, I've listened to some reorchestrated versions of the Golden Sun 1&2 OST. I've also been listening to the Total Annihilation OST, and I will sometimes open up my mod (modulation) collection of chip-style music which is all collected from modarchive.org and from the freeware video game "Lyle in Cube Sector" (you will find all of the tracks from Lyle in Cube Sector at modarchive though). Every once in a while I will also listen to Dubmood's chip-style music. I will also listen to various JRPG soundtracks; ones with high energy and fast drums, or sometimes more emotional ones like Sanctuary from KH. Because I have a thing for cute girl voices, the past 4 days I've pretty much just been listening to the newest album from Dal Shabet (name is Korean; means "Refreshing sherbet"), which is a k-pop girl group. And I've also been listening to familyjules7x's music, which tends to be metal and very awesome, and you can find him on YouTube. A lot of the time I will listen to music found in various animes as well, and I may even listen to some nightcore. Sometimes I'll give Pendulum a listen, or some other good DnB, maybe some chillstep (some Blackmill falls into this category, like "Miracle") or dubstep. Might throw in some Noisia, like the 16-bit Machine Gun remix. It all depends on what I think will be inspirational/motivational at the time. I use music as a means to kinda stay focused/awake or just to make me go "yeah! YEAH! I'm making cool stuff!" Music just tends to keep me excited about what I'm doing, which is why I listen to music most of the time that I'm making anything, Cave Story stuff included, of course.
EDIT:
Randolf reminded me: I have also been listening to Celldweller as I work on stuff. Mainly the Voicians remix of "Louder Than Words", and Drop Wave's remix of "Switchback".