Atmel
AVR offcial site. You find
here AVRStudio (simulator), IAR free assembler, application notes
and datasheets
BASCOM
AVR Basic. A powerful basic for AVR devices,
free demo up to 2K code with IDE and examples.
Fast
AVR Basic. Another powerful basic for AVR devices with IDE
for easy development.
AVR
GNU C compiler
porting of the GNU C compiler to the AVR architecture, probably
the best free compiler
ICC
AVR A powerful
commercial C compiler for AVR (it supports Tiny and 1200 too).
Some features are: support for Tiny and 1200, powerful IDE, application
builder, integrated ISP tool.