Extattr portability?

I'm trying to complete the extended attribute support in
libarchive.  There are a handful of odd issues that arise
which I think I could resolve if I knew of good use cases.

Anyone here actually use extended attributes?  (Note that
ACLs are handled separately.)  What software?  What
information do you store there?

If you could benefit from being able to move extended
attributes between FreeBSD and other systems, I'm
especially interested.

Cheers,

Tim Kientzle