The source code for the original Serious Sam has been released to the public. If you're a modder, this gives you a ton to work with, and if you're not, well you can sit back and expect the mods to start rolling in.
Going open source has proved a major boon for modding and multiplayer communities in the past, particularly Doom's. One can hope the same effect is achieved with Serious Sam.
As is typically the case, technically just the engine used for the game is open source, so you'll still need a copy of the game -- which includes the assets -- to be able to mod it.