Is there an example of an Activity that Responds to an external event?

Bashar,

I think what you'll want to investigate is Asynchronous Server Events.  There is a KB (#272; http://kb.k2workflow.com/articles/kb000272.aspx) that covers this. 

Below is an excerpt from the KB:

"Asynchronous Server Events provide a mechanism for stopping process flow to wait for an external application to programmatically finish the server event before process flow resumes. They behave similar to client events, the difference being that asynchronous server events expect “machine” input from an external system or application, whereas a client event expects “user” input in the form of one or more destination users executing an action against a worklist item associated with the client event."

The Kb article pointed by BOB is your solution.

You can find some other threads talking about that :

http://www.k2underground.com/forums/post/27593.aspx

http://www.k2underground.com/forums/thread/21205.aspx

HTH