Just a quick post to inform, it looks like GOG have started their christmas sales season by marking Duke3D as FREE (unsure if only for a limited time or perma-free) as well as advertising some pretty awesome prices all round.
Today there's a 75% off all bullfrog games:
Populous 1, 1.5 and 2
Dungeon Keeper 1 and 2
Syndicate (n.b. no 'American Revolt')
That bundle is going for $11.92.
There are a good number of games that are heavily discounted too and surely more 'bundle deals' over the coming days, so prepare your wallets and get ready to rock out with the old school games!
EDIT: Some of CD Project RED's flagship titles are on sale too (spotted Witcher 2 for $12, down from $30) and I've added links.