The stack

Building a durable research library from the browser
Zotero Connector
The durable, open-source library every project should start with.
4.0 / 5 store rating7,000,000 usersFree and open source; optional paid Zotero storage
Read full review 
Annotating web pages and PDFs, alone or with a class
Hypothesis
Annotate papers and share notes with a lab or reading group.
4.1 / 5 store rating200,000 usersFree for individual use; paid education tiers exist
Read full review 
Jumping from any page to Scholar results and full text
Google Scholar Button
Scholar results, citations, and full text from any page.
4.6 / 5 store rating3,000,000 usersFree
Read full review 
Explaining and navigating academic papers
SciSpace
Explanations when a paper's methods are the bottleneck.
4.4 / 5 store rating200,000 usersLimited free use; paid plans
Read full review 
Collecting web, PDF, and video highlights
Glasp
Highlights that survive across web and PDF reading.
4.5 / 5 store rating500,000 usersFree tier; paid plan options
Read full review 
Collapsing a pile of open tabs into one restorable list
OneTab
Collapse a literature sweep's fifty tabs into a restorable list.
4.4 / 5 store rating2,000,000 usersFree
Read full review