Are there any open-source trading bots out there? Has anyone published an open-source trading bot that runs on a Bitcoin exchange?
ga bitbot

from the project page:


  ga-bitbot is a distributed genetic algorithm tuned automated mtgox trading system for Bitcoin which includes market data collection, gene client/server, reporting tools, and automated trading.
  
  The intention of this project is to create a high frequency capable trade platform for the bitcoin p2p currency.

bitcoin arbitrage

From the project page:


  bitcoin-arbitrage - opportunity detector and automated trading
  It gets order books from supported exchanges and calculate arbitrage opportunities between each markets. It takes market depth into account.
  
  Currently supported exchanges to get data:
  
  MtGox (USD, EUR)
  Bitstamp (USD, ~EUR)
  Bitcoin24 (EUR)
  Bitfloor (USD)
  Bitcoin-Central (EUR)
  BTC-e (USD, EUR)
  Intersango (EUR)
  Bitfinex (USD)
  
  Currently supported exchanges to automate trade:
  
  MtGox (EUR, USD)
  Bitstamp (USD)
  Bitcoin-Central (EUR) - closed
  
  (under MIT licence)
  
  *TODO:
  
  Tests
  Write documentation
  Add other exchanges:
  icbit
  BitFinex
  Update order books with a WebSocket client for supported exchanges (MtGox, Bitcoin-Central)
  Better history handling for observer "HistoryDumper" (Redis ?)
  Move EUR / USD from a market to an other:
  Coupons
  Ripple ?
  Negative Operations
  use Litecoin or other cryptocurrencies trades