My assembler is written in a fairly quick and hacky way, however it seems to work ok.
It needs PHP5, and you run it like this:
php -f <path to asm.php> <path to .asm file>
It will write a file called the same thing as your .asm file, but with the extension “.hex” on the end.
I’d be really interested if anyone uses this assembler; email me at nabaztag@magicmonkey.org if you do. It’s licensed under the GPLv2, which basically means you can use it how you like, but if you want to distribute it then you have to make any changes publically available and you can’t really restrict what people do with your changes (except that any changes that they distribute must be publically available etc etc). For more info, read http://www.gnu.org/copyleft/gpl.html.