A third Serious Sam game using the same engine, called "The Last Encounter," was originally planned, but abandoned in favor of Serious Sam II, which boasts an all new engine.
On the level "The Citadel" we see a Family Portrait after an elevator. What we actually see is a portrait of the Croteam crew.
On the snow level, the background music is a mixed version of "Jingle Bells"
Sam Stone dons a t-shirt to replace his tank-top from the first game. Strangely enough, the opening cutscene recounts the events of the first encounter with Sam wearing a t-shirt instead of his tank-top. Rumors abound that this was to make the character look a little different from Duke Nukem.
Croteam is indeed caring about the fans of their Serious Sam games. If you look deep in the credits, you can see the names (or actually, nicknames) of the most active members of the Seriously forum (http://dynamic3.gamespy.com/~serioussam/forums).
A few of them were even awarded one of the most wanted prizes in the entire world: a secret room with their pictures. Look for it on a side of the temple, in the first level.
The additional developer, "A Few Screws Loose", helped develop the multiplayer code.
'Serious Sam: The Second Encounter' was named the Game of the Month for February 2002 by GameSpot.com.