I bought a Packet8 VOIP adapter a couple months ago. Just yesterday, Qing tried it in Hangzhou and it worked very well.
There are several independent Voip providers, they are usually much cheaper than the big telco or cable companies (Time Warner, for example, keeps asking me to sign up for the $40/month phone service). I chose Packet8 based on cost ($20/month) and friend’s recommendation. By making certain assumption of your typical home DHCP network, it makes the connection really pain-free.
The quality so far is pretty good, it is hard to tell whether the call is from a conventional circuit or not.