FASM
A good all around assembler, great for beginners because of its simplicity.
http://flatassembler.net/
NASM
A more mature assembler, has more features than FASM.
http://nasm.sourceforge.net/
MASM32
A popular choice for coding for Windows.
http://www.masm32.com/
There are others, but these, I think, are the more better/popular ones.