Ophis is a cross-assembler for the 65xx series of chips. It supports the stock 6502 opcodes, the 65c02 extensions, and syntax for the "undocumented opcodes" in the 6510 chip used on the Commodore 64. (Syntax for these opcodes matches those given in the VICE team's documentation.)
Ophis is written in pure Python and should be highly portable.
If you have questions or comments, email me at mcmartin AT gmail DOT com.
The manual Programming with Ophis is distributed with each download. You can also get it alone.