For a short while, Special:Search showed results from other wikis in a very sensible way: for instance, looking for a word on Wikipedia gave you also a link to the entry on Wiktionary in the same language; searching for a name gave also the link to the person's quotations on Wikiquote; etc. If I remember correctly, they were displayed in a box on the right side of the screen.
This is a feature that, from what I recall, had a universally and greatly positive reaction and then suddenly disappeared. It's quite obviously useful because few readers (or even editors) know the differences between Wikipedia, Wiktionary, Wikiquote, Wikinews... We've missed it for many years now and it should probably be among the highest priority feature requests for our search (after "keep search lights on" but before any design or user experience work).
I don't remember any detail, maybe someone can find links. Looks like it was around 2009.
T49775: Centralized Search Engine ("interwiki" searches)
T55531: Images in Commons don't show up in search when using CirrusSearch
T3837: Search all languages
T62975: Expose interwiki search results in API