https://www.mojeek.com/focus/
Regular search, but include up to 25 domains/subdomains and optionally exclude up to 25 domains/subdomains. No sign up etc, cookie based.
Nice feature, but you need a "Wow! Gee Whiz" article in The Verge and a post in HN to get some buzz. This is great for search geeks but I was once told by Cutts Tim Mayer that "Nobody uses search attributes! Less than 2%!" --maybe a separate page & url will boost it?