Punchbowl - A party planning and greeting card website
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
RSVPify - A powerful and awesomely customizable platform for selling tickets online for your event.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Online-RSVP.com - Beautiful online RSVP for wedding & other events. Create an online invitation, track your guests responses.
Bigtent - BigTent is a web-based organization and communication portal for groups.