Carnation Software - Keyspan USB to Serial Adaptor
MacWise - Macintosh Terminal Emulation - telnet / serial for OS X - Wyse, Viewpoint, VT100, VT220
 
Keyspan
USB to Serial Adapter
  

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.

    "High Speed" USB Serial Adapter
Uses a PC standard 9-Pin serial connector.
This is the least expensive solution since it has just one port and does not require a Mac modem cable.
   

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.
    Macintosh Modem Cable for use with Keyspan Twin Serial adaptor.