With this addon you can make users verify their email before they fill out the registration form. This can be helpful to not have a bunch of unverified users laying around.

Clicking on the register link will show users a popup like this:

Once they enter an email, and click on verify, they see this:

The email the user gets with the code looks like this:

Once the user gets their email code, and enters it on the registration form and fills out any additional fields, they are automatically registered and marked as valid users.