These guides are independently maintained. Much of their content is included in the Ubuntu community help pages, but newer material in these guides may not be.
Ubuntuguide - Comprehensive guide for Ubuntu, since 2005.
Kubuntuguide - Comprehensive guide for Kubuntu, since 2007.
Ubuntu Geek - a wealth of information, but in blog format
Psychocats' Ubuntu Linux Resources - a collection of random tutorials for Ubuntu