Gmod is essentially Roblox's father. It's limited in what you can do but is still really good and has a shockingly active community.

The best Roblox alternative is S&box, but it's not out yet. It's a spiritual successor to Gmod that aims to be a gaming platform that's developer friendly and good to consumers. There's a developer alpha you can join to start making games in it, but the community is not there yet and it's likely a few years away.