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.

CryoutCreations

Quickly and easily swap between themes. Adds a theme switcher to the WordPress Admin Bar with all parent/child themes, filtering and favorites list. Compatible with WordPress 4+ 
Performs transparent theme switch by redirecting to the initial URL on both the frontend and in the dashboard.

Cryout Theme Switch

Quickly and easily swap between themes. Adds a theme switcher to the WordPress Admin Bar with all parent/child themes, filtering and favorites list.

Multisite Switcher vs Cryout Theme Switch