This plugin adds to the signup page of WordPress MU a checkbox for TOS and/or a privacy policy.

Very effective anti-spam plugin that eliminates comment spam, and registration spam. Combines many effective methods for identifying spammers and keeping them off your site. THe plugin is designed to be very light weight so it will not interfere with the operation of your website. Easy to install and default options work perfectly on most sites. Will not lock you out. No Captchas needed. Works with JetPack Protect.

Track referrals with Booking Calendar

Track your AffiliateWP referrals using s2Member Pro Coupon Codes

Add an "Agree to terms" check box on login, registration, or comment forms. Works with BuddyPress, bbPress and MultiSite.

Allow multiple user accounts to be created, registered, and updated having the same email address.

Prevent people from registering with emails that contain a subdomain to help reduce spam.

The ultimate plugin and service to stop and kill splogs in WordPress Multisite and BuddyPress, from WPMU DEV.

An easy way to place a Aweber form on your Wordpress site.

The #1 AWeber plugin to get more email subscribers. Easily add AWeber sign-up forms as popup, embedded widget or sticky top bar.

Botosub is the simple and effective tool for sending newsletters from Facebook and increasing your Facebook users.

Add a "Member Type" option to the BuddyPress registration form.

Logs the IP of the user when they register a new account. View the signup IP by going to the Users menu or by editing that user.

CalculatieTool.com signup and contact form.

Add a Mailchimp signup form block, widget, or shortcode to your WordPress site.

Convert visitors to subscribers with targeted pop-ups, sideboxes, bars and inlines. Works with MailChimp, ConvertKit, AWeber, GetResponse & others

Simple create an event with guestlist and signup.

The DiloBox Subscription plugin allows you to quickly and easily add a signup form for your DiloBox list as a widget on your WordPress site.

It's signin extension from AA Production House.

An addon for the Affiliates Manager plugin to sign up affiliates to your Mad Mimi list