I've used this website a few times now and am in love with it (it's got the picnik feel too). I think it could be a little more user-friendly (i've had to search around for things), but it's very cute, with a lot of options--and free. And it's not just sending an invite--you send the invite and there's a whole event website set up for you and your guests including a map, message board, etc anything you would like basically--send the link to twitter/Facebook friends, post it to your wall, check to see who's looked at the invite, get weekly rsvp summaries....really the options are limitless. And they/you can choose how to remind your guests of the party/event (SMS, email, etc). Go check it out for your next planned event or e-Card you want to send!