YALC is a virtual architecture designed for educational purpose. It models a DLX-like processor and its set of ASM instructions, a compiler from a high level language (C-like), and an IDE with syntax checking and highlighting.
A assembler plugin for Eclipse, to use different assembler-programs (e.g. NASM, TASM or MASM) as back-end compiling. It include linker and debugger controlling, too. And you get a optimiced ASM-File-Editor with this plugin.