There are many useful websites that extract data from Codeforces and give some statistics, problem recommendations, graphs, etc.

I thought that collecting them all in one place would be a good idea. Please comment the websites you find useful :D

List as of now:

• CF-Predictor (Original Blog): Predicts the expected rating change during and after contests.

• Code-Drills (Original Blog): Recommends problems for a list of user & gives statistics on total number of problems solved.

• Codeforces Visualizer (Original Blog): Gives various statistics for a user, and supports comparison between two users.

• CF Statistics (Original Blog): Comparison between 2 users gives the unique problem-set and the shared problem-set solved by the users.

• Codeforced: Gives a contest-wise result of which questions have been solved and which haven't been solved (but attempted). Essentially, a complete list of "unsolved problems" as opposed to Codeforces' last 15 unsolved.

• Virtual Contest Picker (Original Blog): Given a set of usernames, finds contests where none of the users have made any submissions.

• Codeforces Comment/Blog Statistics (Original Blog): Gives statistics per user: No of blogs/comments/upvotes.

• Team Dashboard (Original Blog)

• Codeforces Practice Tracker (Original Blog)

• Codeforces Upsolving Helper (Original Blog): Returns list of all attempted problems for a user that don't have AC.

• CF Notifications (Original Blog): Notifies the user of their verdict on any CF submission.

Some missing sites:

• Most popular users