OzTheGreat

For WordPress Multisite (WPMU) installs, allows Super Admins to easily manage each users roles and blogs from one 
screen in the Network Admin menu.
You no longer have to go to each blog to change the user’s role. It’s also 
much easier to see which sites a user is associated with.

Multisite User Role Manager

Ajax management makes it super easy to add users to a bunch of sites in the network. Does 1 things really well, more plugins should work like this.

Great for multisite user management

Made adding a user to 20 sites a lot easier than using WP's default interface for that (way less clicking around).

Much easier to assign a user to many sites

I spent about 30 minutes looking at different multisite user management plugins and all of them were either out of date, or tried to do too much. This plugin solved the exact issue I was dealing with: adding a user (with a specified role) to all of my client's subsites. 

Thank you very much. 

I wish you all the best,
Kevin.

P.S. the CSS for the admin screen where you add users to subsites was a bit off for me (in Chrome). The table did not fit the modal background (it hung off to the right a bit, and forced me to scroll even though there was plenty of vertical space). It doesn't bother me but it might bother some other users so I figured I would let you know. 

Excellent plugin - it does what it says!

Manage user roles for each blog from a single screen on multisite (WPMU) setups

irenem

A simple tool that allows you to switch between user accounts without the hassle of logging in/out. It allows you to view, review and do what yours does to your site.
<h4>Features</h4>
<ul>
<li>
Switch between user account and to guest user.
</li>
<li>
Inherit all capabilities of the selected account, as if you are that very same user.
</li>
<li>
Helpful for developers, designers, and site owners.
</li>
</ul>

User Switcher

This plugin helps me very much when I need to test custom user roles.

I like it

This is really helpful plugin for testing my user's issue with my site. When I have only admin account I can easily switch to a random users and reproduce their issue. Thanks for the great solution.

Perfect tool for testing users issue

A helper tool to help you switch between user account without logging in and out.

Multisite User Role Manager vs User Switcher