This is NASM - the famous Netwide Assembler. Back at SourceForge.net and in intensive development!
Much news since 0.98.39 was "not available on SF"!!!
0.98.39 *is* available here, and 0.98.40 - with "-f macho" support - is available from Apple.
0.99.03 has just been released - a "work in progress". 64-bit support - bugfixes, speedups, etc. We think this one works... but watch for bugfix releases!