I believe that anybody interested in computing should at least once, contribute to open source projects. In particular the ones licensed with Free Software licenses that respect the four software freedoms. Below are some of latest ones I have contributed recently (since 2021).

Project License Repository
secp256k1 (Bitcoin Core) MIT code
bitcoincom-solidity-swap GPLv3 code
Ethereum web3.py MIT code
Ethereum Solidity GPLv3 code
Arbitrum docs   code
Aragon’s Open Design System GPLv3 code
Polygon CDK Data availability   code
Polygon zkEVM node AGPLv3 code
Polygon zkEVM docs MIT code
Polygon zkEVM js library AGPLv3 code
Polygon-Edge Apache-2.0 code
Polygon-CLI AGPLv3 code
Polygon docs MIT code
Plonky3 Polynomiol IOPs MIT code
matic.js MIT code
Bor - Matic blockchain GPLv3 code
Heimdall - Validator node for Matic PoS layer GPLv3 code
Saturn Network Explorer   code
Filecoin Saturn Multiple code
Filecoin Lotus   code
Filecoin Docs   code
IPFS specs Multiple code
IPFS kubo Multiple code
IPFS desktop Multiple code
IPFS docs Multiple code
IPLD - InterPlanetary Linked Data Multiple code
go-ipld-prime MIT code
Bacalhau - Decentralized computing Apache-2.0 code
Risc0 - Zero-Knowledge VM Apache-2.0 code
Risc0 zeth - ZK block prover for Ethereum Apache-2.0 code
Rain Protocol   code
Stacks Docs   code
Hiro Systems Docs   code
GAIA - Stacks MIT code
Stacks Blockchain Docker MIT code
Stacks Blockchain GPLv3 code
Stacks.js MIT code
Stacks   code
Clarinet GPLv3 code
Clarity REPL GPLv3 code
Clarity Book CC0-1.0 code
Stellar Soroban tools   code
Stellar JS SDK Apache-2.0 code
Stellar Rust Soroban SDK Apache-2.0 code
interBTC and Kintsugi docs Apache-2.0 code
interBTC Clients Apache-2.0 code
interBTC API Apache-2.0 code
CryptoCurrency eXchange Trading MIT code
Mercury Cryptocurrency arbitrage AGPLv3 code

Other projects

Project License Repository
linuxserver GPLv3 code
Opnsense Update BSD 2 code
Opnsense Core BSD 2 code
Webtrees GPLv3 code
Zoneminder GPLv2 code
Privacy guides CC0-1.0 code
doctoc-docker Apache-2.0 code
Big Blue Button LGPLv3 code
Awesome Privacy CC0-1.0 code
darktable.org.es AGPLv3 code
darktable.org CC BY-NC-SAv3 code
darktable GPLv3 code
Exiv2 GPLv2 code
Lensfun LGPLv3 code
Imageio BSD 2 code
LuaAutoC license code
Pixelfed AGPLv3 code
Peertube AGPLv3 code
FSFE Website AGPLv3 code
FSFE CD Front AGPLv3 code
Reuse tool GPLv3 code
OpenStreetMap ODbL code
Documenso AGPLv3 code