I'm Not Going If She Doesn't Go...
Do you know about evite.com? You can send email invitations to a party (many to choose from), and the guests can rsvp online, with a personalized message. All the invited guests can see who is invited and if they are going or not. You can allow invited guests to invite additional people, if desired also. You can have your guests set up a carpool to your party, and leave messages for each other. There are a few other services there too. It's really neat! Quick and easy for busy moms, and free. Save the postage money to spend on brownies for the party.
I love using evite! Make trackign those invitations much easier1
Our Moms Club uses this for all of our activities because it is by far the easiest way to invite a group of people to multiple events. My favorite part - there's a link on the left side of the page to add the event right into my Outlook calendar.