Well if you're looking for a more business-like applications. The L^ane Point of Sale system. It runs Perl/Curses and/or Perl/TK, and uses PostgreSQL. It handles cash drawers and reciept printers.
The Description from the website:
LanePOS is an integrated Point-Of-Sale (POS) and backoffice system for cash register-like computer systems. It allows a business to track inventory, sales, and customers in a single or multi register system. Although LanePOS was originally designed for retail stores, restaurant functionality has been recently added. This system allows any backoffice computer to access the system for reporting or modification, by using VNC (or another X11 "wrapper") running on a register for backoffice functions
Unix - where you can throw the manual on the keyboard and get a command|