Maldita Castilla is a nice, free (CC), cross platform side-scroller (with the Linux version!), made in the spirit of the old school challenging arcade games. It features good graphics and soundtrack and really deserves to be included in the GOG games library.

You can vote here.

* Game trailer.
* Some game posters.
* Original soundtrack.

Like you said it's free just like all other Locomalito's games, but as I see GOG as a sort of Game Preservation Hall, I'd also love them all to be here, if only to receive the attention they deserve.

Go Locomalito!

PS: voted ;)

Not my type, but it looks extremely well made.