It's a bit off topic since it's not a resource for learning, but:

http://tatiyants.com/pev/#/plans

(Postgres Explain Visualizer)

When dealing with query optimization in practice, it helps a lot since it enables a clear, convenient and synthetic view of an EXPLAIN (ANALYZE).

Just yesterday, it helped me improve a big analytic query from executing in 10 minutes to 5 seconds by just adding a small index.