Blockchain
Dune Analytics
https://dune.com/browse/dashboards
- User-created dashboards
- Extra useful for DeFi
Etherscan
- ethereum focused
- has wallet labels, transaction search, smart contracts
Google BigQuery
https://cloud.google.com/bigquery/
- Supports most major chains including Bitcoin, Ethereum, Zcash, doge, etc
- SQL syntax
- very useful for graphs
CoinGecko
- FREE API, with no keys