Here is an excellent resource that lists the various query string parameters that can be used in SharePoint Site URLs.
Thanks to Daniel McPherson's Blog post for this info:
SharePoint Query Strings
My favorite is ?Contents=1. This will display the SharePoint 'Web Part Maintenance Page' which can be useful when you have a Web Part that is causing an error. You can then close the Web Part in this Maintenance Page.
This usually redirects to something like this: