With a Linux box in the basement, a firecracker X10 device, an X10 Appliance Module, and some grow lights, you too can have a poor-man's timer for starting seedlings. Sure timers only cost $20 bucks, but isn't hacking one more fun?
Start with Heyu software. If you are dabbling with X10 and don't have Heyu, you're in for a treat. Heyu will let you get all Bill Gates Home Automatey with very little money. To get Ubuntu/linux to talk to your lamps and whatnot, you'll also need the cm17a, the serial port X10 transceiver, or it's younger USB brother, the cm19a.
I have a few modules hooked up to random lights around the house, and we use one on the Christmas Tree for the holidays. It's simple to get Heyu to talk to your modules through the electrical wires.
Note: My cm17a which is the serial port device that communicates to X10's various modules is NOT plugged into my serial port, but attached to a serial -> USB adapter. I've seen various reports of success/failure with this type of device so you mileage may vary. Personally, it has not been a problem. In my Heyu config file, I'm pointing to my device 'by-id' (this is all one line):
Ubuntu tends to load USB devices in a random order. Once you have this properly configured, you should be able to control your lights from the command line using the device ID you set on each specific module (note the preceding 'F' for firecracker specific commands):
heyu fon a1; heyu foff a2
Boom. I just turned on the grow lights, and turned off my bedside lamp. I wanted the lights for the seedlings to emulate the sun somewhat so they'd go on around 8am and go off around 6pm. From here, the cron job practically writes itself:
# m h dom mon dow command 0 8 * * * /usr/local/bin/heyu fon a1 0 18 * * * /usr/local/bin/heyu foff a1