Designing a URL Shortening service like TinyURL 

Designing Pastebin 

Designing Instagram 

Designing Dropbox 

Designing Facebook Messenger 

Designing Twitter 

Designing Youtube or Netflix 

Designing Typeahead Suggestion 

Designing an API Rate Limiter 

Designing Twitter Search 

Designing a Web Crawler 

Designing Facebook’s Newsfeed 

Designing Yelp or Nearby Friends 

Designing Uber backend 

Designing Ticketmaster