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.