SurroundSend
I got an invitation today via a new Evite-ish service called Pingg. I liked the looks of the invitation, so I clicked through to learn about it it a little, and discovered that the service includes something called SurroundSend, so that users can easily notify others about Pingg events via Facebook, Twitter and other social network apps:

SurroundSend - pingg.com, originally uploaded by Stowe Boyd.
I cooked up an invitation, and had it sent out via Twitter: however, in the Twitter case at least, it doesn’t work the way I would have liked. I wasn’t able to include the Twitter handles of my friends that I was inviting: it simply blasted it out via my Twitter account. Wrong model. But the service seems pretty nice other wise, and supports many forms of notification — some for a fee — like SMS and surface mail.