Here we go:
1x Arduino Uno from Cool Components plus
1x Arduino WiFi Shield
Arduino WiFi BlackWidow
along with this library to easily generate the web pages: WiServer - AsyncLabs Documentation
and some port switching code tied in behind Hobby Robotics » Using Relays with Arduino – Turning on the Lights with a relay wired in as demonstrated.
Have fun and remember that these micros have eight I/O pins that you can use to control lights along with A/D converters etc. You can have it sensing all sorts of stuff like temperature and light level reporting it back to you via the web interface or use it to let the device make decisions like not turning it on if its light.
I'd also take a look at these Scratch boards (just to give you more options, I don't know whether it can do what you want): Scratch | Home | imagine, program, share
I'm going to be using a version of it called Sense when I start my Open Uni course next week, had a quick play and it's incredibly easy to use, though the visual "programming" interface might be a bit too limiting...
EDIT: sorry, I've just taken a look at the website I recommended and it's totally not what I thought it was! Turns out I have my wires crossed, Scratch is a basic visual programming language, not an electronics kit. What I actually have is a customised version of Scratch and an Arduino board. Bonus for me, not so helpful to you, sorry!