There's something to be said about Web services that have been set up to help people coordinate things in the least stressful way as possible. I dig sites like CircleUp (coverage) that offer a way to set up polls, or to solve quick logistical questions within a group, without requiring the creator or the users to agonize over the interface and execution. That's why MyPunchBowl's new checklist feature is pretty much the best addition to a party-planning service yet.
The idea is simple: you, as party creator, make a list of things you need for the party. This list is just for you, in a GTD sense. You can hit one button to add items to your "potluck" list, which is made public to all your invitees. Your party guests can then assign themselves to which items that they'll (hopefully) bring to the party, or add their own items. This helps you avoid making harassing phone calls or e-mails to coordinate the details.
The new checklist feature was launched along with several other updates. The service is continuing its foray into the world of themes with about two dozen Halloween templates and a new duplicate party feature that lets you clone and reuse a party page you've already made--which is helpful if you want to recycle things like a guest list and directions. There's also a bounced e-mail notifier to let you know if your party invitations never made it to your addressees' mailboxes.
I'm still waiting for a feature that will track your recipients' past replies to flag those who say they're coming and never do--and one that flags guests who show up without replying--to let you know when a call, personalized e-mail, or bouncer is in order.