Its definitely not a freeware,
but when i think about any type of architecture diagrams i prefer
visio.
Pros:
ease of use 10/10
extensibility : you can make your own "stencils" and use them in any diagrams
portability: I love the export to "EMF" format. EMF (Enhanced Meta File) is a format supported by XP by default and is a vector oriented format (just zoom !!)
also its support in MS
Word (direct paste) is good which in my opinion is what you are going to resort to, to do the documentation (unless you are a Linux guy)
Export to JPG,Gif, etc.
Cons:
You cant expect it to generate code from uml diagrams (At least I have not discovered this)