External documentation
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List of generic documentation elsewhere on the web.
This does not include documentation for specific software: a link to them should be placed on the wiki page for that software.
Howtos and linux generic documentation
The Linux Documentation Project:
http://www.tldp.org/
HowtoForge provides user-friendly Linux tutorials about almost every topic:
http://www.howtoforge.com/
Distro-specific documentation
Please add the documentation for your favourite distros here.
Debian
Debian official documentation:
http://www.debian.org/doc/
Debian Documentation Project:
http://ddp.alioth.debian.org/
Ubuntu
Ubuntu official documentation:
https://help.ubuntu.com/
See also
30 Best Sources For Linux / *BSD / Unix Documentation On the Web
http://www.cyberciti.biz/tips/linux-unix-bsd-documentations.html