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:
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.
Some missing sites:
- Most popular users
- Most upvoted blogs/comments