TopCoder is one of the original platforms for competitive programming online. It provides a list of algorithmic challenges from the past that you can complete on your own directly online using their code editor. 

Coderbyte provides 200+ coding challenges you can solve directly online in one of 10 programming languages (check out this example). The challenges range from easy (finding the largest word in a string) to hard (print the maximum cardinality matching of a graph). 

Project Euler provides a large collection of challenges in the domain of computer science and mathematics. The challenges typically involve writing a small program to figure out the solution to a clever mathematical formula or equation, such as finding the sum of digits of all numbers preceding each number in a series. 

HackerRank provides challenges for several different domains such as Algorithms, Mathematics, SQL, Functional Programming, Al, and more. You can solve all the challenge directly online. HackerRank also provides the ability for users to submit applications and apply to jobs by solving company-sponsored coding challenges. 

CodeChef is an Indian-based competitive programming website that provides hundreds of challenges. You are able to write code in their online editor and view a collections of challenges that are separated into different categories depending on your skill level. 

Exercism is a coding challenge website that offers 3100+ challenges spanning 52 different programming languages. After picking a language that you'd like to master, you tackle the coding challenges right on your machine (Exercism has their own command line interface that you can download from GitHub). 

Codewars provides a large collection of coding challenges submitted and edited by their own community. You can solve the challenges directly online in their editor in one of several languages. You can view a discussion for each challenges as well as user solutions.