Skype 2.0.0.6 (by Skype Technologies S.A.)
Skype is the next phenomenon from the people who brought you KaZaA.

Ovolab Phlink 3.1 (by Ovolab)
Ovolab Phlink, combined with the Ovolab Telephone Adapter, can answer telephone calls and identify the caller using Caller ID and Apple's Address Book.

BluePhoneElite 1.1 (by Reel Intelligence)
BluePhoneElite extends the calling features of your cell phone to OS X with a simple yet powerful interface.

Send SMS 1.7 (by Alco Blom Software)
How do you send an SMS? Are you using the T9 input method? Are you also wondering why you are using these small keys with numbers on it to enter text while you have your Mac keyboard in front of you?
Then the Send SMS widget is for you.

Hold On 0.6 (by Wooden Brain Concepts)
HoldOn lets you use your build-in modem as a speaker phone to listen to the line while on Hold.