UberConfernce, from Firespotter Labs, now has WebRTC conference calling, as long as you're using GoogleChrome.
As their web site says: "When entering the conference page (either directly taken there if the conference is now, or when the conference starts) a popup will display your connection options. From here you can choose to connect from your computer by selecting the option and hitting ok."
Right now WebRTC is rather new, and somewhat "geeky" but it won't be long. From what I know, UberConference is the first of the conference call services that are making this feature available.