Kailey (trepmal)

Adds the ID of the current blog/site to the Edit Site menu item. Available only to super-admins.

Blog ID in Site Admin Menu

Adds the ID of the current blog/site to the Edit Site menu item.

fritsjan

With this plugin you can simply batch create / add a bunch of sites to your WPMU install (WordPress Network) and use one of your existing sites as a template for the new sites. The existing blog will be cloned exactly including posts, layout, settings, etc. The plugin also takes care of domainmapping the newly created sites. (depends on ‘WordPress MU Domain Mapping’ by Donncha to be installed)
This plugin will save you a lot of time. 
Even adding one site is faster with this pluging compared to the normal way: add site, domainmap it, set everything up.
Update: You can now also use it without domainmapping, so just cloning sites in a batch! Insert the name of the subdir or subdomain you want to make in the place of the url.

Add Clone Sites for WPMU (batch)

Batch add new sites on Wordpress MU / Wordpress Network while using a template site to clone, includes batch domainmapping of the new sites.

Blog ID in Site Admin Menu vs Add Clone Sites for WPMU (batch)