A good set of resources for Atari related - GEM, GDOS, GFA Basic, MC680x0 etc
http://atari.scum.org/aldevdoc.htm
GFA Basic Download Archive
http://www.gfasoft.gfa.net/eng/pub-entry.htm
Ioncannon: 6502 C Compiler
http://www.cc65.org/index.php#Download