NASM, the Netwide Assembler, is a recently written x86 assembler. It can be used to write both 16-bit and 32-bit (IA-32) programs.
32-bit programs can be written for NASM in such a way that they are portable between Microsoft Windows (any 32-bit variant) and Linux, if the right libraries are used.
NASM is currently developed at SourceForge, see http://nasm.sourceforge.net .