Taken from: The WordPress MU Manual
WPMU default setting for plugins is disabled. If you want users to be able to access regular plugins you first need to enable them site wide.
1. Click on Options (Site Admin > Options)
2. Select Enabled for Plugins
3. Click Update Options

Activating and deactivating regular plugins sitewide
There are two options for regular plugins:
1. Activate Plugin Site Wide –activates the plugin site wide i.e. plugin s automatically enabled on all blogs on the site and users can’t deactivate it

2. Activate Plugin Per Blog Basis – allows users to decide which plugins they want to use on their blog i.e. users can choose which plugins they want to activate/deactivate

Now there's no need to pay for a third party service to sign up, manage and send beautiful email newsletters to your subscriber base - this plugin has got the lot.
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No javascript required, no third part chat engine, just fully featured chat right in your own database on your own WP sites - couldn't be easier.
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Simply insert google maps into posts, sidebars and pages - show directions, streetview, provide image overlays and do it all from a simple button and comprehensive widget.
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If you're thinking about starting a paid, or just private, membership site then this is truly the plugin you've been looking for. Easy to use, massively configurable and ready to go out of the box!
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Out of all the WordPress ecommerce plugins available, this has got to be the winner - easy to configure, powerful functionality, multiple gateways and more. A simply brilliant plugin!
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To get a wiki up and running you used to need to install Mediawiki and toil away for days configuring it... not any more! This plugin gives you *all* the functionality you want from a wiki, in WordPress!!!
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It's now incredibly easy to start your own Q&A site using nothing more than WordPress - The Q&A plugin simply and brilliantly transforms any site, or page, into a perfect support or Q&A environment.
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If you've ever wondered how you could offer a paid site management and hosting service, then this is the plugin for you. Offer a freemium or paid service, for any niche you like, it's powered Edublogs.org to success already!
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Would you like to add Facebook comments, registration, 'Like' buttons and autoposting to your WP site? Well, The Ultimate Facebook plugin has got that all covered!
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I have one wpmu install for multi-site/domain
So site activation is running per domain ?
I am not sure if this is the right page to post this but I am looking for a plugin that would do the following:
1) whenever my blog post gets published, the plugin automatically posts the url of the post to social media and social bookmarking sites I have preprogrammed in (with logins/passwords)
2) After it’s posted to the social media sites, it then creates a forum thread on some forums I’ve preselected and entered login info for.
Is this possible? Please email me at the email I’ve supplied for this post.
Thank you in advance. NOTE: I’ll PAY for this custom plugin, not looking for a free solution.
I reckon you’ll be best off here asking for that :)
http://premium.wpmudev.org/wpmu-jobs/