Altwiki by Bakuretsu

What it does
Pick one wiki, or several with priorities. The default is a single wiki. In priority mode the first ticked wiki is asked first, the second only if the first has no article, and so on.
Default: a random working Wikiless instance. Wikiless is a privacy front end for Wikipedia. Altwiki picks a random public instance, skips the ones that are down, and if none answers you stay on Wikipedia. You can add your own instances.
Same language. de.wikipedia.org goes to the German edition of a wiki, never to an English article. Wikis that only exist in one language are skipped for the others.
Only real article views. Edit, history, talk, special pages, files, categories and ?redirect=no links are left alone. Add ?altwiki=off to any Wikipedia link to skip the redirect.

Limits
An article is only found when it has the same title as on Wikipedia. HandWiki titles carry a topic prefix (for example Physics:), so Altwiki also tries the common prefixes.
If the wiki then shows its "this page has no text" notice (the standard MediaWiki missing-article block, in any language), Altwiki sends the tab on to a working Wikiless instance, when there is one. This happens once per redirect and only on pages Altwiki sent you to.
With a single wiki there is no check: the article address is built from the title and you are sent straight there. In priority mode a wiki is judged to have the article when the page answers normally and does not show the usual "no such page" text; a site that blocks the check or answers oddly is given the benefit of the doubt. Sites that handle missing pages in an unusual way (Grokipedia, Baidu Baike, Baike.com, Everipedia) are checked with extra rules that may need adjusting when the sites change.
Altwiki asks the wikis for the page before redirecting, so a redirect can take a moment. Answers are remembered for a while.
Privacy
Altwiki sends no data anywhere except the requests that check whether an alternative wiki has an article (priority mode only) or whether a Wikiless instance is up: the address of that article on that wiki (and, for Wikiless, on an instance). Settings are stored in the browser and never leave it. There is no analytics and no account.