Craigslist

Twitter

Nested Comments

Scheduled Digital Transaction

Hyperparameter Training & Tuning

Audience Segmentation

One Time Password (OTP) with Cache

File Upload and Hosting Service like Dropbox

Photo Sharing Service

Health Check Endpoint Migration

Ad Click Ecosystem

Logging Service

TinyURL (Revisited)

Stock Exchange

Reddit

Online Presence Indicator

Distributed Counter

Dating App like Tinder

Price Drop Tracker like CamelCamelCamel

Pastebin

Distributed Linked List

Weather App with Millions of Smart Device Sensors

Google Calendar

Digital Wallet

Ticketmaster

Flash Sale

Spotify "Top K" Songs

Captcha

Metrics & Alarms Service

Orchestration

Rate Limiter

Get Active Events

Vending Machines

Social Graph

Dashboard

Search Autocomplete

Airline Ticketing

Chat App like Facebook Messenger

Distributed Cron

Distributed OLTP Database

Traffic Violation Detection Service

Superbowl Voting Service

Uber

Tagging Service

Payment Gateway

Online Auction and Bidding Service

Coding Contest platform like Leetcode or Codeforces

Workflow Automation Service

Distributed Tracing

Inventory Tracking & Management Service

Web Crawler / Search Engine

Hotel Booking

Batch Marketing Email Service

Ad Click Counter

Video Upload and Hosting Service like Youtube

Proximity Service