Is there a modern (I mean excluding Ext2/3 as they are obsolete for general usage when there is Ext4 today) free-born (I mean excluding FAT and NTFS) file system working equally good in Ubuntu and Windows (maybe after installing some extra drivers) (or any active project of such)?
There's the Universal Disk Format.
But Ubuntu/Gnome/udev doesn't support it much outside of DVDs. And Windows might not run it from ordinary partitions either without helper tools. But at least there is sufficient kernel support for it be useable as interchange filesystem.
Since it's an ISO standard I'm assuming it's free from discriminatory software-patents.