The PC side of this is already done. I just need to package it up nicely.
When it comes to compilers, there is only free option, SDCC. The other option, Kiel, costs an outrageous amount of money. SDCC works in both Linux and Windows.
Likewise, there is only one option here. Someone named Brennen did a huge amount of work and compiled an SDK that makes use of nearly the entire chip's functions and released it to everyone under an open source license. Combine this with my Wiring library and you get a familiar and portable environment.
I considered integrating this with the Arduino IDE, but then I remembered how awful that is in comparison to just about everything else. Right now I am using Sublime and a make file. It works , although it could use some work to automate things. It works on Linux and Windows. build-essential is required on Linux for make. You can try installing CygWin on windows for make, but I have just been using a bat file called make.bat. Sublime doesn't know the difference.