Here is a forth style assembler for the atmel avr series of microcontrollers. I use it with gforth, but it should work with any ans forth.
This file is the assembler proper; you need it.
There files describe a specific avr part. I've included files for the parts I use. You can make your own if it's not listed here or ask me nicely.
These are driver code for both internal and external peripherals.