Loads here click here
If you do a google search for ip phones you'll find loads more.
I actually use such a system - though it's a proprietary system (as I have no phone line, and it's provided by by bb net connection supplier.
You can also get adapters to plug into a router and use standard telephones - and a router with the adapter built-in - so you just connect a standard phone direct to the router. See some details here click here