Is there an OS X terminal program that can access serial ports?
You can use the terminal command screen to do this!!!

As seen on ServerFault:


  I love using [screen] for connecting to serial consoles, i.e.

screen /dev/ttyS0 19200



Or, if you prefer Mac OS X hints...


  I often have to do router configuration via a console port, so I use a
  Keyspan Serial Adapter to get access. Two problems then present
  themselves: ZTerm is a horrible Mac OS X app. It hasn't been updated
  in five years or so, and isn't a Universal Binary. The developer
  doesn't seem in any hurry to rectify the situation. It is not worth
  the shareware fee in its current form. Minicom requires installation
  of Fink or MacPorts and is overly complex. Solution: Use screen,
  Terminal, and a little AppleScripting.
  
  First, launch Script Editor and type/paste in the following code:

tell application "Terminal"
  do script with command "screen /dev/tty.KeySerial1"
  set number of rows of window 1 to 100
  set number of columns of window 1 to 80
  set background color of window 1 to "black"
  set normal text color of window 1 to "green"
  set custom title of window 1 to "SerialOut"
end tell

  
  Compile and save as an app from within Script Editor, and you have a
  double-clickable application to launch a serial Terminal session. You
  may want to customize this slightly -- you can change the screen
  colors or number of columns or rows. You may also need to customize
  the screen command with a different device name if you are using
  something other than the Keyspan Serial Adapter (do an ls tty* of the
  /dev/ directory to get the right name). 
  
  screen uses Control-A to take commands directed to it. So type
  Control-A followed by Control-\ to exit your screen session. If you
  fail to do this and exit a Terminal session, you'll leave the screen
  session alive and the serial resource unavailable until you kill the
  screen session manually. man screen will show you further commands to
  send to a screen session. 
  
  If anyone can reply with a link to a tutorial on how to wrap an
  interactive Unix App in Cocoa, that would be the next step -- it would
  be nice to do this without involving Terminal. If you prefer to use
  Minicom, you could still use the AppleScript to wrap it into a nice
  launchable app -- use this older hint to find the right command line
  commands.


Many USB-Serial adapters use the chip from FTDI. Install the "Virtual COM Port" driver and look for the proper TTY name in /dev. For example, on a PowerBook G4 it came up as /dev/tty.usbserial-FTALKY8I.