TripIt has the most clever registration system I’ve encountered. Here are the two steps:
1) Take any confirmation e-mail for your next trip—airline, hotel, car rental, whatever—and forward it to firstname.lastname@example.org. (Best is something like your round-trip plane reservation as it has the start and finish dates of your trip.)
2) Go to http://www.tripit.com, enter the e-mail address you just sent the forward from, the password of your choice, and click “Sign up – It’s Free!”
TripIt will display your trip information that it has already drawn from the e-mail you just forwarded. What’s more, you have just completed the full registration process, and by “full,” I mean full. TripIt gleans from your forwarded e-mail all the pain-in-the-nether-region stuff like name, address, airline frequent-flyer number, etc., etc., etc. You are good to go.
What TripIt has done is to make not only registration, but data-entry itself, simply disappear, but they haven’t done so by denying themselves any information. They have only relieved the user of having to do all that typing.