This a thousand times. I have to ALWAYS google that one big query for showing me all the databases. (I almost always connect with pgAdmin)
(I sort of agree with the parent, in that, if you need this in a raw SQL interface, it's a pain. But it shouldn't be a common pain point?)
In pgAdmin I would have to expand every schema to see every table.
Was a bit tired when writing that comment.
(I'm a CLI user, so it's a \dt away. But I Googled screenshots of pgAdmin prior to commenting to see if it was in the UI, and it/was.)