Intel2GAS is a converter that will convert assembler source files written for NASM to files that can be assembled using the GNU Assembler (GAS), on the i386 platform. It provides support for basic MMX instructions as well.
This converter was written for the Hermes project and is now released to the general public. The code inside this converter is extremely flexible, you can write a syntax file to convert between any two assembler languages on a good day.. :)
You can get the source code right here, it uses autoconf and should compile under most UNIXs (maybe DJGPP and CygWin32 as well):
You can also get the following binaries: