This plugin by Cédric Boverie is for WordPress 3 only. If you have a multisite install and if some users manage 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.
Plugin Homepage: http://github.com/cedbv/Wordpress-Plugins