URL Shortener aka TinyURL comprises an array of worker hosts whose number can be varied as per traffic (Scalable) and Memcache as distributed cache for low latencies.
Scalable, Web socket based Messaging Application having one to one chatting and file exchange capabilities. It also shows online user and Send/Read message status
An Auto Complete or Type Ahead functionality can be build on Distributed Tries where each node caches top K terms of that prefix. These top K terms are dynamic and based on data mining of Current trends/ Top news etc.
MVP of Twitter. Architectural choices and twitter feed optimizations as system scales. An overview of celebrity tweet problem and a possible solution to it.