Throwing a party's supposed to show that you're a fun, spontaneous guy, but how freewheeling could you actually be if you're spending an afternoon filling out fields just to pin people down with an invitation? Screw all that, and use Anyvite. Anyvite's a dirt-simple, stripped-down event-invite service hatched by two Boston-based brothers, one a former professional hacker/security expert, the other a tech consultant for several imposing three letter agencies (PBS?). Instead of an exhaustive survey of your personal info, AV only asks for date, time, and guest list; once the invite's sent, you can receive updates, comments, etc, straight to your IM or Gchat, because who doesn't love a window popping up to say, "The Three Musketeers Ride Again. Can I bring Darren too?". Anyvite also spreads the convenience to your friends by allowing them to RSVP straight from their email, and to easily integrate the service with their cell; there's also an "open" event function with public URLs and custom RSVP forms, in case you have no pre-existing friends to spread the convenience to.
Most recently, AV's added a feature that'll pull data from flickr and twitter accounts to create profiles of your guests, so they don't have to spend afternoons creating their own profile, and digging around for a picture where they're not making a spontaneous sex face.