AVaRICE is a program which interfaces the GNU Debugger GDB with the
AVR JTAG ICE available from Atmel.

Use AVaRICE standalone as a programmer, or it can be run as an
intermediary between avr-gdb and the AVR JTAG hardware, allowing one
to use GDB to debug your AVR code as it runs in-system.

WWW: http://avarice.sourceforge.net/