TextFlow
TextFlow makes it easy to review document versions simultaneously, and rapidly produce a final draft. Cut time with the only visual version manager that helps you compare, recombine and merge several versions of a document.
Purpose: Version Management. (Document Comparison).
Operating System: Desktop version under development. Online version is called CompareMyDocs.com. API available for developers.
Price: Personal and commercial editions – The online version is free. 
Development: Nordic River Software AB, Based in Umea, Sweden.
Links:
Introducing TextFlow: flash video – flash animation
Press:
TechCrunch: TextFlow’s Shiny Document Collaboration System
WordsWare blogposts:
textflow-online-is-called-comparemydocs.com