A few people have wanted a hack to create a Stock Market on your phpBB and this has yet to be created. While you wait here is how to make a simple stock market using the Shop Hack.
First of all install the Shop Hack as well as both upgrade files in the installation. Make sure to edit the files.
Once this is completed, go into Admin and make a shop called The Stock Market or whatever you wish to call it.
Then create items that are to be your stocks. They could be real stocks, or ones relating to your forum.
Now every so often (say a week) you can change the prices. It is a good idea to have a forum to post your price changes in and why they've changed.
If you have any questions, feel free to send me a private message, or to check out a demo visit my forum (link in sig)