I don't currently own a computer interface (or any x10 devices for that matter) but I have been looking around with interest. Being a total gadget geek, I was wondering if someone could suggest a good PC interface to purchase.
My personal preference is the ADI Ocelot. It's a relatively inexpensive device that supports X10 and IR. It also allows you to add on additional modules for relays, more IR, etc.
I've recently switched off of the Ocelot to the SmartHome PowerLinc USB X10 module for X10 control. If you are just using X10, it's a very cheap and effective X10 controller.