UI Widget & Component Toolkits

On Web Platform: ¹

• Bootstrap 3 or Bootstrap 4
• Kendo UI for jQuery [free to $]
• Materialize
• Office UI Fabric
• Semantic UI
• UiKit
• Webix [$]

React Specific, On Web Platform: ²

• Ant Design
• Material ui
• Semantic-UI-React

Native Desktop/Laptop/Netbook Apps via Web Platform (i.e. used with NW.js and Electron):

• Photon
• React UI Components for OS X El Capitan and Windows 10

Mobile/Tablet Specific On Web Platform (i.e. used with touch focused UI's):

• Framework7
• Kendo UI Mobile
• Ratchet

ADVICE:

¹ If you need a basic set of UI Widgets/Components start with Semantic UI. If you are building something that needs a grid, spreadsheet, or pivot grid you'll have to look at Kendo UI or Webix. Also, keep in mind that most of these solutions still require jQuery.

² If I was going to build a React app and needed a toolkit of widgets/components off the shelf I'd with Semantic-UI-React and/or Ant Design, or I would accept that fact the some of the components I want to take off the shelf and use have a hard dependency on jQuery.