So one of those annoying things which happen during the user registration process on a website is that of missing the activation email, or the email which is sent by the website to activate your account. If this happens - your account is stuck in limbo. You have registered, so you cannot re-register with the same email, but at the same time, you cannot use the website because your account is not activated. The more annoying problem with Joomla is that there is no way to resend the activation email - at least natively.
But - as with everything else, Joomla extensions to the rescue :)
RegRemider is a component / plugin whose primary intention is twofold - number one is to remind users to complete their registration and second of all it allows you to send existing members reminders every so often so that they once again visit your website. The 2nd function is extremely important if you want to keep your customer base engaged, and if you are reading this article - you have probably experienced the first problem so this component solves this problem for you too :)