The default index file looked for by Apache when a directory is requested is index.php, followed by other popular choices (index.html and others). You can change the default (first) index file searched so that, for example, index.html (like the Joyent default page) is served instead of index.php, by adding the following to a .htaccess file in your document root directory for the virtual server in question:

DirectoryIndex index.html

The order of precedence is from left to right, so you can add other choices to the right of index.html (separated by space), so that other files will by tried if the first one is not available.