AS31 is a free 8051 assembler originally written by Ken Stauffer.
Versions of AS31 found here include important bug fixes and added
features, such as standard intel-hex output format for use with
PAULMON2 and EPROM programmers. AS31 is a good tool for building
small 8051-based projects that are written in 100% 8051 assembly language.

WWW: http://www.pjrc.com/tech/8051/