Katas (Practice Sites)
Includes a facility and resources to practice solving programming problems and sharpen your problem solving skills.
/r/dailyprogrammer
Includes challenges set for beginners, intermediate, and advanced programmers.
Advent of Code
An annual event where a challenge set is gradually released every Advent season (starting from December 1st).
Codechef
A website and a community of coding challengers that offers a wide variety of coding challenges-related activities. Includes occasional contests, programming tutorials, a freeCodeCamp forum-esque Q&A forum, and coding challenges for all programmers of different skill levels.
Personal comment:
One of my personal recommendations. Give it a try.
Codewars
A community-focused programming kata site where users can create their own katas to let other people solve it.
Personal comment:
It's probably one of the most popular kata site out there.
Codingame
A coding practice site where you get to train with visual feedbacks (in other words, playing video games). They also have an active community behind it, too.
Daily Coding Problem
A set of coding problems that's being asked by top tech companies such as Google and IBM is being sent to your email one problem every day. Offers at least a hundred of coding challenges from easy to advanced problems to be solved in your way.
Personal comment:
They also have a relatively recent book on it, as well.
Euler Project
Has more focus on applying mathematical concepts than programming.
Personal comment:
Recommend it a lot, especially if you want to be more challenged.
Exercism
A site that focuses improving you through the mentor-mentee model.
Google Coding Competitions
A site where Google coding competitions take place. Also hosts a variety of competitions and you even have access when a certain coding competition is done so you can take at your own pace.
HackerRank
A site that offers programming challenges along with some beginner-friendly sets.
HackerEarth
A programming challenge site that well, encourages you to take on the challenges in programming. It includes programming hackathons, challenge sets, and a curated collection of programming problems for various technical concepts such as data structures, mathematics, and basic programming.
Pramp
You get to practice some programming interview questions and it's free.
Personal comment:
Very much recommended, also beginner-friendly for challenges (kinda).