Tools
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Assembler:
NASM - Netwide Assembler
http://www.web-sites.co.uk/nasm/index.html
ftp://ftp.simtel.net/pub/simtelnet/msdos/asmutl/

Linkers:
ALINK - Linker
http://alink.home.dhs.org
which redirects to
http://www.geocities.com/SiliconValley/Network/4311/

WLINK - Linker
Used for linking VXD part. Commercial product. Since it's not able to 
make VXDs there's a small patch (xfse.aps) for the "LE" output file.

PATCHER:
APATCH - tiny patch maker
 You can find it at:  http://apack.cjb.net/
                 or:  http://members.xoom.com/jibz/apack/news.htm