Recently, the developers of Serious Sam celebrated the franchise's 25th anniversary. They prepared a gift for the community:
We're sharing something special with you – an old, dusty sketchbook with sketches from the very first Serious Sam game. You can find it right now in the installation folders of The First Encounter and The Second Encounter under the name "Serious Sam Sketchbook".
In addition, the developers announced collectible cards and items that will soon appear on Steam for Serious Sam: The First Encounter and The Second Encounter.