Here's a collection of NASA sounds from historic spaceflights and current missions. You can hear the roar of a space shuttle launch or Neil Armstrong's "One small step for (a) man, one giant leap for mankind" every time you get a phone call if you make our sounds your ringtone. Or, you can hear the memorable words "Houston, we've had a problem," every time you make an error on your computer.
And there aren't many restrictions on how you use NASA's sound bites, but do read over their short legal section — legal problems are no fun. There is a larger collection of NASA sounds over at Archive.org, but it's not the most friendly of sites.