Where to find free open source scripts for Roblox Studio?

The Toolbox inside Studio (the Creator Store) is the obvious place, but be careful with free models from it. Plenty of them hide a backdoor script - look for require() pointing at a long number id, or anything using getfenv, and delete it. For actual open source code the DevForum's Community Resources section is better, and most of the well known modules live on GitHub: ProfileStore for saving data, Knit as a framework, Promise, Janitor or Trove for cleanup, Cmdr for admin commands. Read through anything before you drop it in a live game.