The frontend is the interactive and visual part of a software which is designed to be beautiful and user friendly. Alternative Frontends are those softwares which helps in viewing contents of a different platform with different visuals provided that another (Alternative) backend extracts data from that platform. Since we don't interact with the backend of the software Frontends are what we normally describe as websites or applications.
Cloudtube is an open source frontend website to YouTube . It gives us the ability to watch videos, subscribe to channels, and get the RSS links of channels without the need to log into YouTube. Check
Newpipe is an open source android application and a frontend to YouTube . It gives us the ability to watch videos, subscribe to channels,save and create playlists, view trending videos and also to get the RSS links of channels without the need to log into YouTube. Check
Invidious is another frontend to YouTube that also offers a popular and a trending feed (Unlike Cloudtube). There is also an option to sign in. You can get videos without logging in too. Check
Bibliogram is a website that takes data from Instagram's public profile views and puts it into a friendlier page that loads faster, gives downloadable images, eliminates ads, generates RSS feeds, and doesn't urge you to sign up. Check
Barinsta is an open source android application that allows you to view Instagram's public profiles without the need to login. It also has an option to log in to Instagram from your account. It gives you the ability to download pictures, videos, stories and also to view profile pictures in full screen which the original Instagram application doesn't allow you to do. Check
Nitter is a frontend to twitter. It allows you to search and view public profiles. It also allows you to get the RSS feeds of the public accounts of twitter. Check
Fritter is an android application with the same functionality as Nitter. It allows you to view profiles which are public. It also gives you the ability to follow accounts and save posts to a different section. It provides you with a feed of the posts from the peoples you have followed, which you cannot get from nitter. Check