Invitastic is a cute little website which lets you create, send and manage invitations to events. It has fairly limited functionality at the moment, but a couple of nice features.
The invitations don't seem to be customisable - mine came as an email on a yellow background, which wouldn't have been my choice. I did like the fact that it embeds a link to Google Maps so your guests can look up a map of where your event is being held.
The site tracks RSVPs for you - you can see who's said yes, no or maybe, and who hasn't responded. For Mac users, it offers the bonus of being able to download the event to your iCal (for you the host as well as your guests - the invitation comes with the event as an .ics (Ical) file.
The date format on the website is the confusing American style (11/20/2007) but the invitations themselves express dates as "November 22, 2007" which is much clearer for non-yankees.
I assume they'll add templates to give some choice about how your invitations look - in the meantime it's a barebones WYSIWYG email invitation service, no more, no less.
UPDATE: Turns out that you can customise how the invitations appear online - I missed the two translucent arrows on either side of the invitation on the invitation creation screen which let you cycle through a range of different backgrounds. The email version always comes with a simple yellow background to avoid image display problems in email. Thanks to Hillel from Jackson Fish Market (the people behind Invitastic) for getting in touch to clear that up!