Sad sounding piano instrumentals

the whole album
Harmaa - Harmaa

my favourite song is "Saapuminen"

I can't remember now for sure, but I think someone once told me they're a side-project of Tenhi.
 
Can't be arsed to listen to the song, but I'm going to recommend O'Nocturne by Obtained Enslavement.
 
Haha, fucking awful song name.

If you are only interested in songs by metal bands then all that comes to mind at the moment is

Mar De Grises - Self Portrait No. 1
Opeth - Silhouette

Both are far, far better than the song by Thy Light. They actually have dynamics and don't sound programmed (which I am 99.9% sure this is).

You might also want to check out Michael Nyman, Ludovico Einaudi, Robin Alciatore and Yann Tiersen.
 
Aeternus - Under the Blade of the Dead
Beethoven - Moonlight Sonata (an obvious one)
 
Kimno is the artist name, great stuff.

Saturnus incorporates lots of such piano passages into their music, but it's not solely piano based.

love the piano!