Dialpad Communications
Offers monthly billing and pre-paid PC-to-Phone VoIP services. Also supports broadband telephone adapter for use without PC.
http://www.dialpad.com/

Broad Voice
Save money on your phone service. Use BroadVoice and your Broadband Internet ... Business VoIP Service.
http://www.broadvoice.com/

AT&T VOIP
VoIP from AT&T converts your voice into data and uses your broadband connection to send and receive calls.
http://www.att.com/voip/

Lingo
Use your existing broadband Internet connection to make calls with Lingo's VoIP phone service.
http://www.lingo.com/

SunRocket
SunRocket, the No Gotcha phone company, has Bottom-Line Pricing, Enhanced 911, low international rates and the option to keep your phone number.
http://www.sunrocket.com/

Frontier Online
Get reliable phone service, High-Speed Internet and DISH Network Satellite TV ... Order new phone service, change existing service, get a second line, more. ...
http://www.frontieronline.com/

VoicePulse
VoIP service offering phone-to-phone with device. Various features and calling plans.
http://www.voicepulse.com/

8x8 Inc. - Packet8 (VoIP)
The Voice and VideoPhone over Broadband Internet Phone Company
http://www.8x8.com/

My People
Broadband phone service
http://www.mypeople.com

ZingoTel
Unlimited Calls US/CAN
http://www.zingotel.com/online/en/