My current favorite is LoseIt both on the computer and my android phone. It's very quick use on the fly. A very similar site is My Fitness Pal.
In terms of a computer site that is "full service" and has all the bells and whistles, SparkPeople is by far my favorite. Unfortunately, their mobile app for tracking food is more cumbersome. So I tend to socialize on SparkPeople through their wonderful teams and blogs, but I track my food on LoseIt.com.