[Micrococonut]

Garry Newman, creator of Garry's Mod, is taking a shot at it https://sbox.facepunch.com/. If anybody is going to succeed in dethroning Roblox, it's Facepunch.

[breakfastduck]

Absolutely. Plus Garrys mod pre dates the official Roblox release (the free versions of gmod at least), so Roblox kind of copied him.