Writing multi-platform applications

[Mike Jeays]

Apologies if this is a bit off topic.

I have been pushing the idea at work that we should write all
applications so that they are OS-agnostic. They should work equally well
(as far as that is possible...) on both the Windows platform, and any
reasonable Unix.  I have quoted as good examples OpenOffice, GIMP,
Apache, MySQL, as proofs that it can be done.

This seems a good way to preserve our investment in home-grown software.
At present we are facing significant costs in migrating 25-year old
applications off our IBM mainframe, and we often need such long
lifetimes.

What libraries and other methods are used to ensure this works with
minimal changes in highly graphical applications such as OpenOffice? Can
anyone direct me useful places to research it further?

Thanks in advance.