Here’s a site i came across for creating UML Sequence diagrams online by just describing the actions (actors, messages, etc) in text.
http://www.websequencediagrams.com/
This currently seems to have only limited functionality, but should be enough for starter. The ‘planned features’ list in the site seems to promise more in the future.
Update: meanwhile, one of my friend sent another interesting tool which reverse engineers the .NET assembly (yes, it’s a .NET specific tool at the moment) and generates sequence diagrams. It is available in codeplex here.
(Again) it seems to be in its early stages of development, but quite promising.
I still feel the need for such tools to be integrated to VS as does the class diagrams (you know the changes made to class in code are synced back in the class diagram, I like that behavior).
PraveenNet 
