I like Programming Praxis: https://programmingpraxis.com/ https://programmingpraxis.com/ There's a pretty extensive collection of problems there now, and they

I like Programming Praxis: https://programmingpraxis.com/ https://programmingpraxis.com/ There's a pretty extensive collection of problems there now, and they span a variety of topics, from basic CS stuff (e.g., implement a stack or a binary tree), to cryptography, number theory, and trivia (e.g., write a quine, calculate the date of Easter, or solve a Sudoku puzzle). Note that the chronological list of exercises isn't up to date.