A WMF-maintained self-contained javascript library to fetch Wikipedia summaries / Wikidata descriptions / Wikitionary translations (and maybe even Commons images) for a given string and display them in a configurable popup window would be awesome. That could then be used as a bookmarklet, in browser extensions (most of those are written in JS so they just need a trivial wrapper around the library), or in web pages as a functionality provided by the site owner.