user323: Has anyone published an open-source trading bot that runs on a Bitcoin exchange?

user1181: You can try this one also https://github.com/rokj/bitcoin_dealer.
From the README file on the repo:
Bitcoin dealer is simple Django program (trading bot) for trading bitcoins currently only on MtGox exchange.
Bitcoin dealer supports MtGox API version 1 with following currencies: USD, EUR, GBP, PLN, CAD, AUD, CHF, CNY, NZD, RUB, DKK, HKD, SGD, THB, JPY and SEK.
Although program is really simple, it supports stop orders and maybe something more, but I am not really familiar with trading techniques and jargon.
Program is most useful for those who do not have time to watch price of bitcoin all the time, have a PC running all the time and know how to write few "Hello worlds" with two if cases.
Also, the link is not dead at time of editing (21/07/2013).

user50: I wrote a simple market maker bot in Python: https://github.com/chrisacheson/liquidbot
This software allows you to place orders on MtGox market.

Liquidbot uses ezl's wrapper code to interface with MtGox: https://github.com/ezl/mtgox

Aricie runs a hosted market maker bot platform: https://dnnbitcoin.aricie.com/default.aspx

user153: 
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



323: <p>Has anyone published an open-source trading bot that runs on a Bitcoin exchange?</p>
 1181: <p>You can try this one also <a href="https://github.com/rokj/bitcoin_dealer" rel="nofollow">https://github.com/rokj/bitcoin_dealer</a>.</p>

<p>From the README file on the repo:</p>

<p><em>Bitcoin dealer is simple Django program (trading bot) for trading bitcoins currently only on MtGox exchange.</em></p>

<p><em>Bitcoin dealer supports MtGox API version 1 with following currencies: USD, EUR, GBP, PLN, CAD, AUD, CHF, CNY, NZD, RUB, DKK, HKD, SGD, THB, JPY and SEK.</em></p>

<p><em>Although program is really simple, it supports stop orders and maybe something more, but I am not really familiar with trading techniques and jargon.</em></p>

<p><em>Program is most useful for those who do not have time to watch price of bitcoin all the time, have a PC running all the time and know how to write few "Hello worlds" with two if cases.</em></p>

<p>Also, the link is not dead at time of editing (21/07/2013).</p>
 50: <p>I wrote a simple market maker bot in Python: <a href="https://github.com/chrisacheson/liquidbot" rel="nofollow">https://github.com/chrisacheson/liquidbot</a></p>

<p>This software allows you to place orders on MtGox market.</p>

<blockquote>
  <p>Liquidbot uses ezl's wrapper code to interface with MtGox: <a href="https://github.com/ezl/mtgox" rel="nofollow">https://github.com/ezl/mtgox</a></p>
</blockquote>

<p>Aricie runs a hosted market maker bot platform: <a href="https://dnnbitcoin.aricie.com/default.aspx" rel="nofollow">https://dnnbitcoin.aricie.com/default.aspx</a></p>
 153: <ul>
<li><h1><a href="http://code.google.com/p/ga-bitbot/source/browse" rel="nofollow">ga bitbot</a></h1>

<p>from the project page:</p>

<blockquote>
  <p>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.</p>
  
  <p>The intention of this project is to create a high frequency capable trade platform for the bitcoin p2p currency.</p>
</blockquote></li>
<li><h1><a href="https://github.com/maxme/bitcoin-arbitrage" rel="nofollow">bitcoin arbitrage</a></h1>

<p>From the project page:</p>

<blockquote>
  <p>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.</p>
  
  <p>Currently supported exchanges to get data:</p>
  
  <p>MtGox (USD, EUR)
  Bitstamp (USD, ~EUR)
  Bitcoin24 (EUR)
  Bitfloor (USD)
  Bitcoin-Central (EUR)
  BTC-e (USD, EUR)
  Intersango (EUR)
  Bitfinex (USD)</p>
  
  <p>Currently supported exchanges to automate trade:</p>
  
  <p>MtGox (EUR, USD)
  Bitstamp (USD)
  Bitcoin-Central (EUR) - closed</p>
  
  <p>(under MIT licence)</p>
  
  <p>*TODO:</p>
  
  <p>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</p>
</blockquote></li>
</ul>
