Just had an email through from gog as they're running a weekend sale with 60% off 23 certain titles. Prices from $0.79 to $5.99 each.
Personally I bought Legend of Grimrock in the past - for anyone old enough to have played and loved Dungeon Master on the Atari ST :)
Many other titles are very highly rated with reviews to read. Also don't forget that all titles are DRM free.