It's On Meet - Unlock who you want to meet by accepting a drink or not
Online-RSVP.com - Beautiful online RSVP for wedding & other events. Create an online invitation, track your guests responses.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
RSVPify - A powerful and awesomely customizable platform for selling tickets online for your event.
FriendsTonight - Meet new friends to join your friends, tonight
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets