Between the personal calendar functionality of a tool like the new Google Calendar and the invitation-driven architecture of Evite, there's an organizational gap. When you want to set up a group meeting and figure out which days people prefer (not just which ones show open in their calendars), who's bringing salad and who's bringing lasagna, and other organizational tidbits that make a get-together work, most calendar and invitation apps fall short.
The new Web service Grouptivity is designed to help you set up these group events. It's essentially a polling engine. You set up your event and the questions online, and people reply via e-mail forms. Its templates are designed around social events: book clubs, baby showers, HOA meetings, and so on.
It is not a calendar system, but it is a handy tool for figuring out whether your book club prefers Rowling to Calvino, and it can also be use for recurring events. You could set it up to automatically send reminders and sign-ups for the snacks for a monthly club get-together.
To be even more useful, I'd like to see Grouptivity (or work-alike functions) built into a calendar system, such as Outlook, Google Calendar, Yahoo Calendar or one of the other clever new online calendars: 30 Boxes, Airset, CalendarHub, Spongecell, Trumba, and so on. Or have it built into an invitation system such as Evite. But like other new online consumer services, Grouptivity is free and fairly useful, so it's worth trying out if you're struggling with the organizational minutiae of a social gathering.