Generally while making a design document of a process, we
use some UML Diagrams (e.g. Process Flow diagram, Class Diagram, Event Sequence
Diagram etc.) to explain the functionality the system.
With Microsoft Visio, we can draw any kind of UML Diagrams. A
60 days trial version Microsoft Visio can be downloaded at:
http://visio.microsoft.com/en-us/TryBuy/TryVisio2010forFree/Pages/Try_Visio_2010_for_Free.aspx
But what is the alternative of Visio if you don't want to
buy. There are some good UML diagram builder sites available that let you
design freely your Class Diagram, UML Diagram, Event Sequence diagram etc. and
let you also to download those diagrams on your local system after finishing
your drawing.
Check the following sites:
For UML Diagram: http://creately.com/Draw-UML-and-Class-Diagrams-Online
For Event Sequence Diagram: http://www.websequencediagrams.com/