Awesome

• Codewars - Codewars is a collective effort by its users. They are creators - authoring kata to teach various techniques, solving kata with solutions that enlighten others, and commenting with constructive feedback.
• Ruby Koans - must have
• Project Euler - Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve.
• Codility lessons
• Practicing Ruby - Delightful lessons for dedicated programmers
• Rubeque - Solving ruby koans and problems from easy to deadly hard
• Ruby Warrior - Learn Ruby through a triumphant quest of adventure, love & destiny
• Coding Mystery - Coding Mystery sends you on a journey of escape-room-inspired mysteries full of coding puzzles, programming challenges, and problem-solving.
• CodinGame - CodinGame is a challenge-based training platform for programmers where you can play with the hottest programming topics
• Ruby Quiz - Ruby Quiz is a programming challenge for Ruby programmers in the spirit of the Perl Quiz of the Week