FOSS cross-platform data drive encryption

[Sam Kuper]

Forgive the cross-posting. I've already asked this question on
superuser.com, but I figure that the sets of MacPorts-Users mailing
list subscribers and superuser.com users might not overlap very far.
The question is:

Is there any dependable FOSS that provides hard disk encryption such
that a data drive (i.e. not a system drive) can be moved between
Linux, OS X and Windows systems, and can be read from and written to
on each of those platforms?

To tailor the question for this list, let me edit that as follows:
"... any dependable FOSS *available via MacPorts*..." :)

Many thanks,

Sam