My Additions : - A2OJ Enhancer — If you use A20J Ladders to practice question, then this extension is absolutely lifesaver. It automatically marks all the questions that you have already done. so that you don't have to check each and every one of them manually.
- Competitive Companion — if you use visual studio code, use this extension with cph(cp helper) plugin. This extension automatically copies your test cases to your ide. Look for its manual to use it with other ide's.
- Competitive Companion 2 (for codechef) : copies all you test cases in single click
- KONTESTS : Competitive Programming Contests Calendar. Just add it to chrome and never miss any of the competitions.