ProgrammableWeb's API Directory has a list of 62 real time APIs including the most popular: A tie between the Twitter Streaming API and Thrutu API. Real Time APIs can aid developers in producing applications pertaining to monitoring, travel, user verification, search, data validation and more.
Twitter has started early beta testing for a new User Streams API. Currently the service is just for desktop clients, but it could eventually fuel other types of applications that need access to data in realtime. Similarly, the stream concept may be one we see used by other services.