Hackerrank.com : I have already written a review about this great and fun website in another blog post. Check it out http://aprogrammersexperience.blogspot.com/2017/05/review-hackerrank-1-great-site-for.html
codeforces.com: It regularly arranges contests where the problems are set by gradual difficulty levels. The problems are later stored in an archive along with their discussions. This is a great website for those who have already learnt the syntax of C, C++ and Java.
codechef.com : It is also like codeforces with a cute looking interface. But here, the problems come along with subtasks. It means you will receive partial points if the solution passes some of the test cases. This feature is absent in codeforces.com. So I recommend trying out codechef first before proceeding to codeforces.
devskill.com : This is similar to codeforces but the problems are a little bit easier.
hackerearth.com : This is an interesting website. It has the features of codechef. It also has a fun challenging arena where you duel with another programmer to solve problems as quickly as possible. It regularly arranges various hackathons and idea competitions too. Competing in these hackathons/idea competitions will really broaden your coding horizon.
geeksforgeeks.com: This is not a website for competitions. It is about teaching you about algorithms and their solutions. It is a great guide for your algorithm courses. It explains the solutions of many important, easy and hard algorithms.
tutorialspoint.com: This is a great website to learn a language quickly. They provide numerous examples when explaining an aspect of a programming language. It is your one-stop whenever you need to know something about a programming language. Whenever I find myself confused about syntax or can't remember the syntax I just search through the extensive examples of this website and I am sure to find my answer explained in an easy way.