A ROBLOX script builder is a game where you can run code in-game. This is a place for basic to very advanced scripters. Script builders are a common hangout for scripters.
Script builders are different now than they were in the past. Commands have changed and are different from previous script builders. For example, if you wanted to run a script you would use a command like so:
Also, there is a change in commands for local scripts. They run like so:
You can also run code using HTTPService . Here is an example on how to run a server-sided script:
You can also run locals similarly:
Most common way to store code for use in a script builder is using Pastebin .
New script builders have what's called a sandbox. A sandbox is a environment made to execute unsigned code, meaning user generated code. If you try to kick a player, the sandbox blocks the kick. A sandbox is a very important and a part of modern day script builders.
A script builder sandbox isn't perfect, as there are many people creating what are called bypasses. Bypasses allow the user to execute code outside the sandbox, allowing things such as kicking a player. Bypasses are usually made by advanced scripters and are usually kept a secret.
A breakout is a piece of code that allows access to the main environment, which means a user can access code that is ran in the game by intercepting commands such as "h/" or "hl/", allowing the user to see their code when the code is ran.
Although, sandboxes may have their weaknesses it is not very complex to fix those vulnerabilities, if the creator of the sandbox knows what he's doing.
Update: Many script builders are being required to filter interfaces made by other people. This means having a script that looks for messages, hints, and anything in a graphical user interface to ensure that it is filtered so that the game follows the ROBLOX community guidelines
What Happened to the Old Script Builders?
The old script builders used loadstring, which is disabled by default. Client-sided loadstring is not allowed, but server-sided loadstring is. That is why the old script builders will never work again. Anaminus's script builder was open source, until its closure in 2014, the year that ROBLOX patched loadstring. Ever since there have been countless users creating new script builders using Anaminus 's code, which doesn't work, even though they claim client-sided loadstring works.
List of Working Script Builders
Some of these may be under review
Voidacity's Script Builder
Anti's Public Script Builder
MasterKelvinVIP's Script Builder (Under review as of 1/27/2017)
MasterKelvinVIP's Backup Script Builder
Nexure's Script Builder
Pkamara's Public Script Builder
AVeryNerdyDude's Script Builder (Closed)
PenguinAnonymous's Script Builder 
Collaboration Bob The Builder ( Not done )