For PIC compilers check out this site:
http://www.gnupic.org/
In response to the statement:
"As mentioned as well, you can program 68xx chips in C : They are the family that has powered Macs from the outset."
Actually the 68XX family was used in the original Apple Computers. The 68XXX was used in the Mac. The new Mac's use the Power PC chip.
Also if you have someone on the team that can contact some of the chip vendors you might be able to get your hands on some demo boards and software. Ask around with people that do this kind of thing for a living. Often vendors will give out the demo boards and software to try to get the engineers to use them in their next designs. I have a PIC demo that I have intentions of using to develop robotics app's with that I acquired this way.
As far as finding freeware, the most popular micros are the derivatives of the 8051 from Intel and derivatives of the 6800 from Motorola. Both of these architectures are available from multiple vendors and both have been around for many years.
Gene F (Firmware design engineer, new product development, Schlumberger)