Gary Chou taught me a trick a year or so ago that I have now used a bunch of times with fantastic results. Email out an invitation with a link to an RSVP form in Google Form and get an automatic spreadsheet of results in Google Spreadsheet.
Here's how you do it:
1) Create a Google Form. I like three text fields; name, email address, and if you are bringing someone
2) After you have completed the form, grab the link by clicking the send button which generates this box
3) Then compose an email with a link to that RSVP form and ask everyone who is coming to fill out the form. That will take them all of twenty seconds to complete.
4) In the upper left of your completed form, you will see this menu
Click on the "Responses" link and you will be taken to a Google Spreadsheet that will fill up with RSVPs as your email goes out and folks complete the form.
That's it. You have a list of people who are coming. You can use it to send emails with updates, you can use it to check people off at the door, etc, etc.
It works great. Give it a try next time you are doing an event that has a big list and requires RSVPs.