Game Feature: Activity Feed
In capture the flag games in Halo, the game notifies all users whenever flag activity takes place (when a flag is picked up, when it is returned, etc). This motivates all players in the match to turn their attention towards the current status of the flag.
When displaying the activity feed on the mainpage of a website players can see instantly when they log on what people are currently doing on the site. If, for example, a player sees that someone else has unlocked a new achievement, this may motivate the player to obtain the same achievement as well.