If you have one of the newer Macs with no serial ports and want to direct connect to your host with a serial connection, you will need a USB to serial adapter.

If you want to dial using a phone line, you need a modem.

NOTE: Keyspan is now owned by Tripplite.
Here are the drivers:

Download the Lion Beta Driver from Tripplite

All Keyspan drivers here

Note: If you are using a non-Keyspan usb to serial device with Lion, you can most-likely get it working by using a generic pl2303 driver here...
http://xbsd.nl/2011/07/pl2303-serial-usb-on-osx-lion.html

Keyspan Twin Serial.
A little more expensive but you get two serial ports.
Uses standard Mac modem cable connectors which means you need a Mac modem cable also.

The Keyspan Adapter plugs into your Mac's USB port.
The other end of the whole assembly plugs into a RS232 cable going to your host computer.
Note: the "whole assembly" drawing shows an older style single-port adaptor.

Zoom 56K V.92/V.90 Modem (USB Mini External)

If you just need to dial using a modem and phone line, you can get this modem (instead of a USB to Serial Adaptor)

The Zoom Model 3095 USB Mini External is a V.92 analog voice faxmodem that combines a highly efficient design, easy installation and high performance in a low-cost product. With its small size (2.8 inches X 1 inch X 0.8 inches), light weight, and integrated USB cable.