Worldbrain is a manager and annotation tool for web content. For me, it's the best I've used so far. The main features are:
* Organizing your links trough tags and collections;
* Searching and filtering these links trough the Memex's dashboard;
* Indexing;
* Annotation tool / highlighter;
* Backup (to a hosting service or offline);
* Nice UI (although it has some things to improve);
* FOSS.

For me, the main issues are:
* Instability: I don't know how the tests are done, but may be almost exclusively automated and/or few/none manual testing are done in Firefox before releasing an update. Twice it happened that Memex broke in the last 3 months or so. This time, in version 1.5.0 (Jan 6, 2020), the main features of tagging, putting links in collections, etc stopped working.
* Can't rename tags (it's possible to rename collections though);
* Can't tag files in general (PDFs, images, etc);
* Searching at "Add Tag(s)" or "Add To Collection(s)" only consider first term/word of tags.
** Eg.: You have a tag named "my tag" or "my-tag". If you search for "my", it will find it; if you search for "tag", it won't find it.
* Filter uses logical disjunction, not having an option to use conjuction instead;
* Can't reposition sidebar to left side;
* No mobile app (yet);
* No dark mode (yet).

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Updates:
2019-12-08
* added 3 issues;
* removed 1 issue: "Issues to backup if adblock is enabled".
2020-01-10
* added 1 issue: unstable;
* edit rating: from 4/5 to 3/5.