Very recently an unknown piano piece by Joh. Brahms was found. Opus posthumus of course. He wrote it when he was 20 years old. It'is called 'Albumblatt' (since he lived in Germany).
It can be found at my wife's website http://www.pianolesleeuwarden.nl
There's an English version of this site (upper right). Search at 'the music', if you'd like to have the tune.
Score engraved with Overture of course.
There are more scores overthere, by the way.