Gnanasekaran Loganathan

Activation mail sent to the new user email once a user registered. The user must activate the account by click activation link in addition to a username and password to log in successfully the first time.
This plugin does not currently work with multi-site. Multi-site uses an activation key system for new registrations already.

User Activation By Email

Require users to click the activation link to access the site which is emailed upon user registration.

Andy Peatling

techguy

Disables the activation email and automatically activates new users in BuddyPress under a standard WP install and WPMU (multisite). Also, automatically logs in the new user since the account is already active.
Possible Future Features: 
-Option to turn off automatic login 
-Option to not disable email 
-Redirect options after account creation
Known Bugs: 
-Doesn’t do the automatic login if you allow blog creation during the user creation in WPMU (multisite)

BP Disable Activation

Disables the activation email and automatically activates new users in BuddyPress under a standard WP install and WPMU (multisite).

User Activation By Email vs BP Disable Activation