What's a good website for logging cycling trip data?
Strava is the de facto standard these days.

Has some nice social features, like segments where your times are compared against others, friends and messages.

Training Peaks is more of a training log than a social platform, has some nice features for planning and talking to a coach.

Polar, Garmin, etc. have their own home made solutions. I've not personally used the Polar one, but Garmin Connect always feels half finished, I only use it to sync to Strava and Training Peaks.

And of course, there are a million others, but I think these are the biggest options.