When you are developing theme you need to view it before making it live or if you want to show the demo of the themes you need to have control on your WordPress and for this there are different plugins available, out of which i have collected only 8 useful plugins. If you know more do share with us.
Allow users to switch which theme they use on your WordPress, WordPress MU, or BuddyPress site. Includes a widget for easily putting the theme-switcher as a list or dropdown in your sidebar.
The plugin is great for theme development as you can tweak the template online without breaking stuff for visitors.
Theme Test Drive WordPress plugin allows you to safely test drive any theme on your blog as administrator, while visitors still use the default one.
This plugin allows you to quickly swap between user accounts in WordPress at the click of a button. You’ll be instantly logged out and logged in as your desired user.
This plugin adds a splash page to your blog that lets visitors know your blog is down for maintenance. Logged in administrators get full access to the blog including the front-end.
6. WordPress Theme Demo Plugin
This plugin was designed to help web designers easily demo their WordPress themes.
7. Raw HTML
This plugin lets you use raw HTML or any other code in your posts. One way to use it is to wrap a part of your post in special tags (below) to prevent WordPress from converting newlines to HTML paragraphs, escaping apostrophes and so on.
8. TDO Mini Forms
This plugin can add themed custom posting and editing forms to your website that allows your readers (including non-registered) to contribute.