Programming competition / online judge sites

• USACO Guide
• codeforces
• atcoder (and somewhat buried, their language list with available libs/modules)
• CSES
• cp-algorithms
• kattis
• Project Euler
• leetcode
• uva
• code chef
• geeks for geeks
• Peking Online Judge
• Zhejiang Online Judge (dated, but >3k problem set)
• Ural State University
• sphere

Sites with frequent contests:

• codeforces
• atcoder
• code chef
• leetcode

For kattis, I found and forked kitty to tweak it for Rust. It's a nice tool for downloading test cases and submitting via CLI. For atcoder and codeforces, there's oj.