Skype

The desktop app that we all use is on iPhone. It can now do VOIP calls over 3G as well. Unlimited US/Canada calls is just $2.95 a month!

Fring

Fring is a nice IM client that supports multiple IM services including GTalk, Yahoo, and Skype. It can do VOIP via Skype as well as with a SIP account from a third party provider.

iCall

iCall is available for Desktop and iPhones. They have both a pay as you go plan as well as monthly unlimited plan.

Truphone

An IM client that allows messaging with GTalk, MSN, Yahoo, and AIM users. Provides low cost Wi-Fi based domestic and International calling.

Bababoo | Just Dial. The Way It Should Be.

Bababoo knows when your mobile is connected to the Internet to provide low and free rates. No internet? No problem. Bababoo is smart enough to pass your call to your carrier so your call is placed no matter what.