Event scheduler can be used for planning of recurring events.
Example: If you are in a small sports club, you could plan weekly games. Members will be able to join or cancel, so other members can see who participates in the game.
You can plan vacation schedules, where no events will be scheduled.
Last but not least you can notify users that have not yet accepted or declined the event by email.
This plugin is available for download to be used on your WordPress self-hosted installation.