OpenTable lets you search for dining options by city or restaurant, and it has detailed information to help you make a choice. Most importantly, it's easy to make a reservation for any size party at a specific date and time, and the site shows you alternative options if your desired hour is booked up - great for popular choices like the aforementioned Gilded Truffle.
Thanks to Dining Rewards, there are even good financial reasons to become a frequent OpenTable user. Every time you honor a reservation, you get points towards cash back from OpenTable. You typically get 100 points per reservation (though it can be more) and 2000 points earns you a $20 reward. It's like being paid to eat out.