The OVA file is nothing more than a TAR archive, containing the .OVF and .VMDK files. Easy!
Using Evergreen ILS for example:
~ $ file Evergreen_trunk_Squeeze.ova
Evergreen_trunk_Squeeze.ova: POSIX tar archive (GNU). I’ts possible to use the tar command to list the contents
~ $ tar -tf Evergreen_trunk_Squeeze.ova Evergreen_trunk_Squeeze.ovf Evergreen_trunk_Squeeze-disk1.vmdk
Simply extract those things…
~ $ tar -xvf Evergreen_trunk_Squeeze.ova Evergreen_trunk_Squeeze.ovf Evergreen_trunk_Squeeze-disk1.vmdk