Steal other people's wordpress plugins

If you can get into their index of plugins /wp-content/plugins

Is there anyway to download their php files to use their plugins?

Normally not. When u request the php, the webserver interprets the file and return html. So I would not hope for that. I would say the only possibility is a misconfigured webserver, where mod php is not enabled or installed. It is not very astonishing to have access to the index, many people forget to disable directory index. But most plugins are available here in the dl section, are you missing anything in particular?

A plugin that opts in readers to my list when they comment.

Great idea! I found that plugin:

http://wordpress.org/extend/plugins/subscribe-to-double-opt-in-comments/

Some tweaking should do it.

That's not the plugin but there are many others that do the same job as the one i listed.