Atmel AVR Microcontroller Languages

C / C++ Compilers

GCC
https://godbolt.org/ See the assembly result for several processors of your code in several high level languages.
http://winavr.sourceforge.net/ WinAVR™ (pronounced "whenever") is a suite of executable, open source software development tools for the Atmel AVR series of RISC microprocessors hosted on the Windows platform. It includes the GNU GCC compiler for C and C++.
http://medo.fov.uni-mb.si/mapp/ Port of the gcc for linux
- http://www.nongnu.org/avr-libc/ AVR Libc : a high quality C library for use with GCC on Atmel AVR microcontrollers.
- AVR Options in GCC
- http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function%20Attributes the __attribute__ attribute for AVR code compiled by GCC
- http://gcc.gnu.org/install/specific.html#avr how to configure GCC to generate AVR executables.
http://www.iar.com/ewavr IAR Systems, IAR Embedded Workbench™ including highly optimizing C and C++ Compiler and IAR C-SPY® Debugger for all AVR devices. The "KickStart" version is free but limited to 4K compiled code and very limited support.
http://www.imagecraft.com ImageCraft ICC-AVR. 45 day trial is then code size limited. $200 full version as of 2007.09.04
http://www.hpinfotech.ro/ CodeVisionAVR C from HP InfoTech (no relation to Hewlett Packard) 90 to 150 EUR as of 2007.09.04
http://www.dunfield.com
http://www.dunfield.com/products/dks.htm Micro-C Development Kits $99 each as of 2007.09.04
http://www.jennaron.com.au/smallc/smallc.html Small-C for the AVR by Ron Kreymborg. Free, open source.
http://www.micro-ide.com/ AVR Atmega128 or 32 C compiler with GUI IDE, supports debugging via LCD in addition to standard AVR Studia. Supports Micromega uM FPU hardware floating point co-processor.
http://www.rowley.co.uk/avr/ Rowley Associates CrossWorks for AVR. Windows and Linux versions. $150 to $1000
http://www.spjsystems.com/cavr.htm C cross compiler with Visual Code Generator $225 as of 2007.09.04

Pascal

http://www.e-lab.de/AVRco/index_en.html Very powerful Pascal and libraries for the larger AVRs. free code limited 4k. 600 to 1200 EUR as of 2007.09.04

BASIC

http://www.mcselec.com BASCOM AVR compiler 80 EUR as of 2007.09.04
http://www.fastavr.com/ FastAVR BASIC compiler includes support for Graphic LCDs $99 as of 2007.09.04
http://www.rhombusinc.com/Bascom-AVR.htm BASCOM AVR compiler $96 as of 2007.09.04

FORTH

http://www.forth.com/embedded/index.html Forth, Inc SwiftX FORTH. $400 to $1300 as of 2007.09.04
http://www.ram-tech.co.uk/avr.htm RAM Technology Systems FORTH