Custom Weapons that are compatible with "Custom Weapons Management". I'm not exactly sure if ALL Custom Weapons can be used by it, or if some of them just weren't made compatible with CWM. Do note though that, just because it is compatible with CWM, without the latter, you can still use these Custom Weapons as Resources for your own Maps (I can't really check that out, but it's a safe bet for me to say they can be used in the "Serious Sam 2017 Editor").

The Gamemode "Shopping Encounter" heavily uses these, allowing you to buy them ingame, but there are also other Mods that take from this large pool of Custom Weapons (I may have to add them here as well, for consistency). Note that Shopping Encounter can have Custom Weapons from Custom Campaigns, but other than being linked in this Description, right below, I don't include them in the proper List here... for reasons, since they happen to be mostly something else, like the "Samus Aran Player Models" that also add the "Arm Cannon" on the side.

Other Mods that have compatible Custom Weapons (unless some were included with Shopping Encounter to begin with, I don't know)(these don't alter the core experience in any seemable way):
[Resource] Quake Weapons and Items
Quake 3 Arena: Fusion
Red Day: Episode One
Samus Aran [player model & weapon resource]
Serious Strike