What are some alternatives?
When comparing Collaborizm and DownToMeet, you can also consider the following products
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Ticket Tailor - Ticket Tailor is an easy to use event ticketing platform that puts you in control.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Agorakit - Agorakit is web-based, open source groupware for citizens.