fasm is short for the flat assembler, which is an assembler (as the name says). It was creted by Tomasz Grysztar, and is still under development (but is very stable).
I like fasm for several reasons, here is few:
It's fast and small.
It's basically 1:1 (if you con't use any macros).
It can produce PE files as well as plain binary, ELF, COFF, COM, MZ, and more.
It's not limitd to windows (you can use it in linux, and thre is a port for MenueOS (an OS created in fasm)).
It's still under development (discovered bugs tends to disapear :).
It's syntax is (IMO) the right syntax:
mov eax,variable ; give eax the address of variable
mov ecx,[variable] ; move the dword at the address of variable to ecx
For more information, please take a look at the official site, and the official support forum. With in the packages there is file named fasm.txt (or fasm.pdf, if you downlaod the windows GUI version), it contains a little intro to fasm, it's syntax, and some intro to some of the instructions it supports. Further more there are examples in the packages.