Recently, i upgraded my home PC. Apart from many others, the one feature which i was waiting to have was the 'Wake up from S5' feature. 'S5' is the ACPI spec name for the powered off state of a computer.
What this feature makes possible is - in the BIOS settings, one can set a time at which the machine should power itself on and save the BIOS settings and poweroff the machine. The machine will wake up on its own at the specified time. This feature is present on all the newer(released in the last 6-8 months, at least) Intel motherboards. Don't know about Gigabyte and others though.
The use case for me is - I have BSNL's DataOne broadband which is free from 2-8 am. Instead of keeping the machine running from 10pm or so, i can have it start just before 2. Saves ~4 hours of power and machine usage for every night that i download. Quite useful...