CSS Swither (Manual and Automatic by condition).
Add-on to WPML to switch off language filters on specific pages.
Adds a dropdown list of the sites with every site owned in a network to quickly switch between them.
Allows admins or all visitors to switch the blog theme. Has a restricted preview mode for your clients and includes a widget.
In a multisite configuration, add a dropdown menu in administration to let you switch between sites admin.
Switch between the backends of your WPMU blogs with one click
The DaySwitcher plugin let you choose a schedule for displaying a WordPress theme.