Jeremy Pry

Force all of the users on your Multisite Network to login at the main site address.

Redirect Network Login

Redirect the the wp-login.php for sub-sites to the main site

Joshua Parker

If you are running a WPMS (Multisite) blogging network, the issue with most login widgets that you add to the main site’s sidebar is that it doesn’t pull the user’s own blog info. That is why the WPMS Sidebar Login Widget was created. Instead of the user trying to remember the login page of their own site/blog, when a user logs into your main site, the widget will conveniently contain links to navigate to 3 different pages of the user’s blog: dashboard, new post page, and profile page.
Features:
<ul>
<li>Link to Network Admin page (only shows if user logged in is a super admin)</li>
<li>Link to user’s dashboard</li>
<li>Link to user’s new post page</li>
<li>Link to user’s profile page</li>
<li>Link to forums page</li>
<li>Subblog registration detection</li>
<li>Custom CSS settings</li>
</ul>
To make sure it works, activate the <a href="https://wordpress.org/extend/plugins/user-switching/" rel="ugc">User Switching</a> plugin on the main site only to switch to a different user.

WPMS Sidebar Login Widget

Adds a sidebar widget to the main site of a WPMU/WPMS install.

Redirect Network Login vs WPMS Sidebar Login Widget