SerialPort X 1.1 (by Altered Horizons Computing)
SerialPort X is a Scripting Addition that allows access to an internal modem or standard serial ports from AppleScript.

Simple Pager 1.2 (by james Sentman)
Simple Pager sends numeric pages to your dial up pager via your modem.

MacDialer 3.0 (by From Concentrate Software)
MacDialer, speed dial for your digital hub.

Dial Phone From X 1.0 (by Automated Workflows)
Dial Phone From X is an AppleScript-based solution that can be used to automatically dial the phone using your computer's modem from within a variety of applications.

goSerial 0.2alpha (by furrysoft)
goSerial is an application that lets you talk to serial devices.

Ricochet Modem Scripts 1.0 (by Thomas Tempelmann)
Ricochet Modem Scripts is a set of two Modem Scripts for Ricochet modems (both for the original 28kbps and the new 128kbps modems).

ABDialer 2.0 (by Macron software)
ABDialer is a smart software utility to automate the process of dialing phone numbers stored on the Mac OS X Address Book contacts.

Modem Light 1.1 (by HernanSoft)
Modem Light lets you have a global floating window with real time information about the PPP connection status, you also have a graphical representation of the lights of a real modem.

Serial ToolKit 1.0.3 (by Deep Sky Technologies, Inc.)
Serial ToolKit is a 4th Dimension plug-in which provides complete access to the serial port, a vast improvement over the native functionality available in 4D's programming language.

Silica 1.2 (by Greg Weston)
I just wanted to know who was calling.

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.