Amazon has a serious deal for Serious Sam fans with the Serious Sam Serious bundle. Seriously.
For $20 dollars you'll own the entirety of the Serious Sam games available - that's 6 games plus some DLC - all available digitally so you don't even have to put on any pants.
Serious Sam 2 Serious Sam 3: BFE Serious Sam Double D Serious Sam HD: The First Encounter Serious Sam HD: The Second Encounter - Legend of the Beast DLC Serious Sam HD: The Second Encounter Serious Sam: The Random EncounterSo I lied, the first Serious Sam game is missing in it's original form, but you do get the HD variety. Almost every game supports co-op except for DD and The Random Encounter. Go get it while the getting is good, and support Co-Optimus in the process!