The old (Unix SVR3?) standard, architecture-neutral format for object files and executables. Today ELF has mostly superceded COFF in modern UN*X systems. However, DEC/Compaq's OSF/1/Tru64 still seems to use this antique format!
Microsoft Windows uses a subtly incompatible version of COFF as its principle linker architecture. This means no UN*X tools will work with these COFF files, making development on Windows even less fun than it could be. Why a company whose flagship product is supposedly based on DLLs would chose the worst possible format for them is something of a mystery.