MVP of Twitter. Architectural choices and twitter feed optimizations as system scales. An overview of celebrity tweet problem and a possible solution to it.
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.
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
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.