Adding a few that I find handy:

!so - stackoverflow.com

!su - superuser.com

!hn - HN :)

!php - PHP manual

!ss64 - ss64.com (CLI reference)

!g - Google

!i - DDG images

!gi - Google images

!imdb - IMDb

!rt - Rotten Tomatoes

Maybe useful: for Amazon and Wikipedia at least (probably others), you can get country/language specific versions by appending the 2-letter ISO code:

!wde - German wikipedia

!wes - Spanish wikipedia

!ade - search on amazon.de

!auk - search on amazon.co.uk

etc.

The ones that I find myself using a lot

!gm - google maps

!sr/!sro - type in subreddit (add o for old reddit)

!so - stack overflow (i usually use it with [tag])