68HC11Asm 68HC11Asm is written by Mark Coniglio . I haven't used it much, but it does have a Mac-like interface and looks quite good. The package available here includes documentation. Both versions come with source code. I also have a copy here .
Source code is included.
Note that Apple has made MPW freely available . There are a couple of Metrowerks CodeWarrior plug-in assemblers for the 68HC11. Another CW 68HC11 assembler was written by Paul Myers and is available through his Web page . The Metrowerks Compiler Plug-In API is pretty straightforward, too. I've also used Zterm which is another excellent shareware program.