Updated: June 9, 2011
WP Interactive allows admin users to run PHP code directly via the WordPress Admin to assist with development of plugins and themes.
More documentation coming soon, but the plugin is stable and very useful.
You can download it at WP Interactive’s Github page. There are currently no plans to submit this to the WordPress plugin repository. I think it might be a little too dangerous to offer up for n00bs to download. No offense, n00bs, but this plugin can do some serious damage to your install if you’re not careful.