The one I'm using on Windows is vasm: http://sun.hasenbraten.de/vasm/
vasm is a portable and retargetable assembler to create linkable objects in various formats or absolute code. Multiple CPU-, syntax and output-modules can be selected.
Many common directives/pseudo-opcodes are supported (depending on the syntax module) as well as CPU-specific extensions.
The assembler supports optimizations (e.g. choosing the shortest possible branch instruction or addressing mode) and relaxations (e.g. converting a branch to an absolute jump when necessary).
Most syntax modules support macros, include directives, repetitions, conditional assembly and local symbols.
It supports many CPUs, among them several flavours of the 6800 family:
6800 family (6800, 6801, 6803, 68HC11)
It can create binary files without any header or format, or platform-specific object files.
It is also actively maintained. I'm using it for 68k family with great success.
