If you're building a HTPC or simply have a PC that only you use then you may want to configure it to automatically log you in rather than having to log in yourself when the machine boots. Many machines these days use GDM as the login system (Ubuntu and most distros using Gnome).
This is a nice simple change, firstly open your GDM config file, this is normally /etc/gdm/custom.conf
Now add the following lines to the config file:
Replacing 'YourUserName' with your actual username. If the [daemon] section already exists then add the lines there, don't create an additional [daemon] section.
If you're using GDM3 rather than GDM, the process is a little different. Instead the file will probably be /etc/gdm3/daemon.conf
instead. The lines you will want to place in here are:
AutomaticLoginEnable = true
AutomaticLogin = YourUserName
Now all you need to do is reboot or restart GDM for the changes to take effect.