Goxtool is a trading client for the MtGox Bitcon currency exchange. It is designed to work in the Linux console (it has a curses user interface). It can display live streaming market data and you can buy and sell with keyboard commands.
Goxtool also has a simple interface to plug in your own automated trading strategies, your own code can be (re)loded at runtime, it will receive events from the API and can act upon them.
The user manual is here: http://prof7bit.github.com/goxtool/