Using The Terminal - Basic guide on using the command line
Troubleshooting - Where to start when things don't work
Logs and Scheduling
LinuxLogFiles - Information on system logs, application logs, and syslogd
CronHowto - guide to cron, to schedule commands
NautilusScriptsHowto - enhance nautilus by adding scripts
Gnome Computer Menu - How to add missing disks in computer:///
Nautilus_Scripts - Using nautilus scripts to extend the functionality of nautilus.
Eye Candy - changing wallpaper & themes, as well as other functional visual enhancements.
Users and Permissions
AddUsersHowto - Add users to your system
BroadcastAlertWithSound - Send pop-up alerts with sound to users
FilePermissions - Understanding and using file permissions
Grub 2 - learn about the grub boot manager
GrubHowto/ChangeDefaultOS - Change the default operating system that boots in Grub
Boot Services - Disable start of unnecessary services during boot.
Boot Text - Spice up your boot text (font resolution and color)
UpstartHowto - Configure your own services to start at boot, or change when upstart services are started
KernelHowto - make a custom kernel for your system and other changes
Time & Date
ChangeTimezoneHowto - change your timezone
Speeding up Ubuntu on Low End Systems: - Making the desktop interface snappier on machines without much memory
Building a custom kernel - Low-level tweaking.
Customize Guest Session - Tips on customizing the guest session feature
BackupYourSystem - How to backup your system
Software Management - All guides regarding adding/removing/updating software