Turn a Raspberry Pi into an oscilloscope. Probes attach to the GPIO pins.
Repairs that you can carry out at home on bricked iPods.
A massive online database of default passwords for networking hardware, embedded devices, and operating systems.
How to debug removable devices under Linux with the Gnome desktop.
An excellent blog post on what to do if your MySQL database server isn't working right because an InnoDB table has been corrupted. I've used this process myself and it works like a charm. Knowledge of MySQL in particular and RDBMSes in general is required.
How to enable a CUPS printer that says it's disabled.
/usr/bin/enable <name> or /usr/sbin/cupsenable
/usr/bin/disable <name> or /usr/sbin/cupsdisable
What to do if your Windows box gives you the "NTLDR is missing, press any key to restart" error message, and refuses to boot.