An easy to use customizable captcha with a honeypot that keeps 99 % of the daily spam away. Never use reCaptcha again.
Based on the q2a point system, user scores get saved per month/year and rewards can be granted to your winners. Highly motivating.
Facebook-like / Stackoverflow-like notifications on your question2answer forum that can replace all email-notifications.
The plugin counts every question, answer and comment as +1 activity point for each user. Let your users know who of them is most active!