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