Simply Silly Software is a development company, which was probably established sometime in the 90's by Joe Wilcox, and are one of the three companies to be involved with the Duke Nukem 3D expansions. The other two being WizardWorks and Sunstorm Interactive, both of which are still active. Simply Silly Software's most notable product is the 1997 Duke: Nuclear Winter expansion for Duke Nukem 3D. They also developed the D! frontend for Doom, Heretic, and Hexen, known as the Doom DeathManger replacement Doom/Master, along with the Doom/Heretic to Hexen pwad converter CONV2HEX, before selling them to WizardWorks for inclusion on the D!ZONE and H!ZONE discs.
They also developed the Shadow Warrior front-end "Shuriken" and the Blood network front-end, "BloodBath". Aside from developing Duke: Nuclear Winter, as well as Doom, Heretic, and Hexen add-ons, details on the company are scarce, which is unusual considering they made an expansion for the immensely popular Duke Nukem 3D. They were also working on an expansion for Shadow Warrior entitled Deadly Kiss. Due to so little information being available, it's unknown if the company still exists but they are most likely defunct as nothing has been heard from them since. However, despite their inactivity, unlike WizardWorks and Sunstorm Interactive, there isn't any information to confirm if they are defunct or not.