This outfit http://www.perceptiveautomation.com/indigo/ produces software for X10, Insteon, Powerlinc and other protocols / controllers that use a browser interface on Mac / PC / Linux / PDA / Phone, etc.
I used an early version of a related niche product (Sailing Clicker) a few years ago to allow me to run presentations / graphics displays remotely from a mobile phone (passé now, high-tech at the time )
No connection with them other than as a user of a related product in the past and I'm sure there are others
Can be done easily, how far you want to go depends on your budget!!
look at www.letsautomate.com who sel laods of x10 gear (dont buy from america, different voltage levels will cause an unnessicary and costly bang!!). No affiliation to letsautomate, just a happy customer for years.
The actual modules to turn on/off a light/appliance is easy enough, what is important is the controller, and this is where you can spend just afew quid, or a a couple of hundred.
Have a look at the site, and particularly the controlers indlucing the cm11, homevision, ocelot and even comfort alarm panel!! All are web controlable in some for or another, some requiring dedicated pc, others having a built in web server.