Serious Sam HD: The Second Encounter

Serious Sam HD: The Second Encounter is the HD remake of Serious Sam: The Second Encounter, and like its predecessor, it has a surprising amount of textures that are clearly stretched out versions of original Serious Sam art. All of the unused stuff in the first HD remake is included with the game, and it has a few extra unused things as well.

”Buy now!” splash screen
Included with The Second Encounter files is a texture for “Buy this game now!” splash screen. There's only one demo for SS:TSE HD, downloadable from Steam, but this one's not seen in it, and is in fact, unused.

Unused Player Models
Four unused player models can be found in the game's files; Soldier, Cobalt Cinderella, Green Greta and Wytee William. They can be reenabled through the editor. Steam avatars featuring these characters can be found on HD TSE's official Steam page.

The Soldier character appears to be from a Serious Engine 3 tech demo shown to a Croatian games magazine, mimicking Doom 3 as a "you don't need to be one of the big US devs to do this!" sort of thing.

These characters were enabled for player use via a limited edition "Serious 8" DLC pack released via the 2011 Steam Summer Camp Sale, along with the pre-order Ugh-Zan player model.