cedbv

Don’t Work in WordPress MU. WordPress 3 only. 
Usefull only if you have a multisite install and if some users manages more than one blog.
This plugin adds a simple dropdown menu in administration header that allows you (and every site users) to switch 
between administration of every site they are member of (and so work for super-admin, admin, contributors,…).
Work best if the plugin is Network Activate.

Multisite Switcher

In a multisite configuration, add a dropdown menu in administration to let you switch between sites admin.

Mitch

WP Maintenance Switch makes it easy to turn on maintenance mode while you are modifying your website.
This light-weight plugin is designed for small operations, such as switching a theme.
If you would like to set up your own custom maintenance page, please <a href="faq/" rel="nofollow ugc">see the FAQ</a>.

WP Maintenance Switch

The button to switch the maintenance mode is so convenient that this extension deserves 5 stars.

Great litlle plugin

A light-weight tool to turn on maintenance mode with just one click.

Multisite Switcher vs WP Maintenance Switch