I've used POD as documentation. I've used HTML as documentation.
I've used *roff as documentation.
I've used LaTeX as documentation. I've used PDF as documentation.
I still use the latter if I need to write something for a customer
and it needs to look "fancy" (I write it in LaTeX, and translate
the dvi to PDF).
But nowadays, I swear by plain ASCII documents. It's the only
true platform independent format (yeah, there are non-ASCII
platforms out there). Anyone can read it. Anyone
can edit it. And just before you think "plain ASCII sucks, I
need markup/images/whatever", realize that RFCs have been written in plain
ASCII for more than 3 decades, and will be written in plain
ASCII for quite some time. I can read RFC 1 (which was written
in 1969 and contains "images") as easily as the newest RFC.
Plain ASCII rocks.