Hello Amos.

look at the Metalink assembler.
Its free.
You can download it
from the Atmel or Philips HP.
Its also upward compatible
to the Keil assembler.
The Keil has better macro capability
and a separate linker for linking
with C code together.
Also you can download from the
Keil HP an evaluation version
limited to 2kB and without floating
point library.

See:
http://www.atmel.com
http://www.philips.copm
http://www.keil.com

Peter