If you have any email address, you've received your share of online party invitations with clip-art-level graphics and a general 1998-ish feel. Someecards, a site with actually good e-cards, makes your invitations fun to read and easy to manage.
If you've used Someecards' greetings before, you know they lean toward the arch, the ironic and the surreal, and they're generally far more smile-inducing than a cartoon purple party had with a bunch of confetti jammed into a background image. Their invite service brings a similar kind of snarky humour to organising parties, and guests get a very usable page linked from their invite email. The most useful feature I noted off the bat was a quick auto-generated Google Maps link, and no need to sign up for Someecards' services to reply or leave a comment. You can give certain guests the ability to invite others, and designate certain folks as "honorees" that won't see a guest list or time details, if you'd prefer.
Someecards' invite service is free to use, requires an account to send and manage invitations. Some invites have commercial sponsors.