Had a hard time finding a straight answer for this. It's so simple on FreeBSD. Of course you can use kermit or other complicated programs, but if you have uucp on your system (most do) a simple command like this will get you hooked up to COM1:
cu -l /dev/ttyS0
If you get a message saying Permission Denied, try running it as the uucp user:
su -c "cu -l /dev/ttyS0" uucp
The escape keys for cu is ~^D (ie: tilde, Ctrl-D) similar to ssh and other programs.