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Mustafa Uysal

AppThemes

On a multisite installation, the admin bar has a menu item called My Sites.
If you’re not a big fan of the admin bar on the front-end, try this widget instead. If the user is not logged in, he won’t see the widget at all.

My Sites Widget

A widget that displays a list of sites that the current user has access to.

Brajesh Singh

This is a plugin for wpmu/wpmu+byddypress powered websites, where network administrators can limit the number of blogs a user can signup. 
It is pretty simple and adds an option to NetworkAdmin->Dashboard->network settings page, where you can limit the number of blogs. No additional frills required.
What It does
It adds an option to the Network settings page when you are logged in as site admin of the wpmu site(or wpmu+buddyppress site),Look at the bottom of options page,and You will see a text box like this asking for number of blogs allowed per user.If you set it to zero(which is the default),It will not restrict the blog registration then. 
For any support or any questions,please leave a comment at my blog
<a href="http://buddydev.com/buddypress/limit-blogs-per-user-plugin-for-wpmu/" rel="nofollow ugc">http://buddydev.com/buddypress/limit-blogs-per-user-plugin-for-wpmu/</a> 
And yeh did I mention <a href="http://blog.strategy11.com/" rel="nofollow ugc">Steph</a> has been very kind in reporting bugs and suggesting fixes.Many Thanks to Steph.

Limit Blogs Per User

This plugin is for WordPress Multisite and/or WordPress Multisite+buddypress based social network.It limits the number of blogs a user can create.

My Sites Widget vs Limit Blogs Per User