LeetCode
One of the leading platform for coding skills. It offers a plethora of algorithmic challenges that range from data structures to databases.
Coding Challenges
Smart Contract Engineer
The LeetCode for Solidity engineers. Look for The Coding Canuck in the Top 100 leaderboard.
Capture The Flag Challenges
Ethernaut
Ethernaut is a web3/Solidity based wargame played in the Ethereum Virtual Machine. Each level is a smart contract that needs to be 'hacked'.