1. A method for suggesting videos to a viewer of a given video, comprising: using a computer comprising a processor and a memory for: creating feature vectors for videos served by a video server, a feature vector for a video describing other videos co-watched with the video; training a ranker for the given video based at least in part on the feature vectors, the trained ranker adapted to produce a ranking score for a candidate suggestion video based on a feature vector of the candidate suggestion video; identifying a plurality of candidate suggestion videos from the videos served by the video server; applying the ranker for the given video to feature vectors of the plurality of candidate suggestion videos to produce ranking scores for the plurality of candidate suggestion videos based at least in part on the feature vectors of the plurality of candidate suggestion videos; identifying a set of suggested videos for the given video from the plurality of candidate suggestion videos responsive to the ranking scores; and providing the set of suggested videos to the viewer.