Available for some time in test mode, Flybys is a (now more prominently displayed) feature from Strava that allows you to replay multiple runs of users of the platform. It is available for all activities, even your training runs, but is more fun for full races were there are much more users matching your data.

On the screenshot you can see the following (texts from the Strava help page)
Playback Controls Play/pause, and step through time. Change play speed and seek to any time in your activity.
Activity List A list of other Strava Athletes you crossed paths with (+ the main activity). Check to add to the playback.
Flyby Locations See exactly where you passed by others on your activity.

The columns have the following meaning
Correlation: Percentage of time that this activity was close to the primary activity. Correlations over 30% are considered group activity matches in Strava.
Spatial Correlation: Time independent similarity of activity to the primary activity. An activity travelling the same course will have a high spatial correlation regardless of the speed or start time of the activity.
Distance: Total distance of each activity in miles.

See a live example from my run on the Thessaloniki Night Half-Marathon last week: http://bit.ly/1kfJVEJ