WordPress Theme Switcher Reloaded is an updated version of the venerable Theme Switcher by Ryan Boren, and Theme Switcher Widget by Jared Bangs. This plugin was sponsored by Themebot and coded by kingler. The plugin is currently maintained by 72pines.

Features:

• Opens themes extremely fast / less load on the server (it would sometimes take 30 seconds or more to load a theme with the old theme switcher plugin. Now it only takes a second or two every time.) This version is ideal for WordPress theme developers who want to host their own theme demos or for anyone who wants to reduce load time and load on the server.
• Support for hosting thousands of themes due to increased efficiency.
• Comes with Widget to add theme switcher to sidebar without modifying code on themes.
• Configuration options for theme list or theme drop-down. Also, you have a choice to display long names (theme name from stylesheet) or short names (theme name from directory) in the widget.
• Supports existing hooks for calling theme switcher via code for themes that already use the themeswitcher hook (although using the widget is preferable.)
• Themes are selected using the unique theme directory name rather than the theme name in the stylesheet (no more broken theme URLs when there are duplicate theme names).
• WordPress themes can now be checked for Valid XTHML and CSS with the theme switcher URL.
• Support for English, Simplified Chinese, Italian and Esperanto.