Free MCS-51 Assemblers:

Dedicated:
ASEM-51 (DOS and Linux): http://plit.de/asem-51/
AS31 (DOS and Linux): https://www.pjrc.com/tech/8051/tools/as31_old.html
ASM51: Original Intel assembler - unknown licence
Metalink ASM51 (DOS): http://web.archive.org/web/20040628060354/http://www.metaice.com/ASM51/Files/ASM51.zip

Multi-target:
ASxxxx (DOS and Linux): http://shop-pdp.net/ashtml/asxxxx.htm
AS (DOS and Windows): http://john.ccac.rwth-aachen.de:8000/as/
Asl "Multi-target portable assembler": http://linux.maruhn.com/sec/asl.html