Awesome Building Blocks for Web Apps

Curated list of standalone features to be integrated into web applications: Get full features built by others instead of having to write everything yourself.
If you like these building blocks, check out componently.com, a marketplace where developers can search, compare and integrate web application building blocks.
Contents
- UI Elements
- Exit Intent Popup
- Auth
- Booking
- Notification
- Social
- Chat Bot
- Chat
- Monitoring
- Content Management
- Consent Management
- Media
- Form
- Marketing
- Changelog
- Telecommunication
- Social Media
- Search
- Feature flags
- Reviews
- Related awesome-lists
UI Elements
- Tailwind UI - UI components, by the creators of Tailwind CSS.
- Webix - JavaScript UI library and framework for displaying data in different ways.
- Highcharts - Interactive charts for web.
- Webatoms - JavaScript bridge for Xamarin.Forms.
- Creative Tim - Themes for React, Angular, Vue, Lavarel and HTML5.
- Syncfusion - Data visualization components and PDF Viewer.
- CoreUI - Vue admin template.
- UIfort - UI design systems for all major frontend frameworks.
- Semantic UI - Open source UI component library for React, Meteor, Ember and Angular.
- Design Revision's Shards Pro - Design blocks for web or mobile apps.
- Themesberg - Bootstrap templates, themes and UI kits.
- Treact - React templates for landing pages, built on Tailwind CSS.
- Creative Designs Guru - Themes for React, Nextjs and Eleventy, built with Tailwind CSS.
Exit Intent Popup
- Wisepops - Contextual popups, banners and bars for mobile and desktop visitors.
- Listagram - Interactive opt-in form for collecting email addresses.
- Poptin - Popups and forms that can be integrated anywhere in the website.
- Embedery - Popups to engage visitors.
Auth
- Auth0 - Authentication management with native support for Angular, React and VueJS.
- Clerk – Complete user management UIs and APIs, purpose-built for React, Next.js, and the modern web.
- Firebase - PaaS from Google, including authentication management.
- Okta - Authentication and Authorization solution with support for Angular, React and VueJS.
- Onelogin - Authentication focussed on company internal applications.
- Microsoft Azure Active Directory - Authentication management in Azure, based on Microsoft accounts.
- AWS Cognito - AWS authentication management. Frontend integration is part of AWS Amplify.
Booking
- Calendly - Stand-alone booking solution.
- Zoho Bookings - Online scheduler integrated into Zoho Suite.
- Acuity Scheduling - Appointment scheduling with extensive API.
- 10to8 - Booking tool with many features in the free tier.
- SuperSaaS - Appointment scheduling with focus on group scheduling.
- SimplyBook.Me - Booking tool with its own booking directory.
- OnceHub - Scheduling via ScheduleOnce and meeting management via InviteOnce.
- YouCanBookMe - Online scheduling for teams.
- OnSched - Scheduling API for marketplaces & applications.