Studio Plugins Building:
qCmdUtl by Quenty https://www.roblox.com/library/142314093/qCmdUtl-Streamlined-building I use this very frequently to resize multiple bricks at once in one direction (regular roblox resizing with multiple bricks selected resizes it at all directions). MoveFirst also helps a lot to move large models very precisely. Spawn a normal brick next to the model, select that normal brick and THEN the model, then use MoveFirst
Mirror Models by JetCrusherTorpedo https://www.roblox.com/library/151648132/Generate-Mirror-Image-Preliminary-Union-Support Mirrors a model so you don't have to build the same thing over again just backwards lol (think like a double staircase, build one side and then mirror it with this plugin)
Part to Terrain by Fastcar48 https://www.roblox.com/library/261634767/Part-to-Terrain I usually build basic environments out of bricks because it's easier to work with, then turn it into terrain with this plugin, then perfect the terrain using the roblox terrain editor
Model Resize by DaMrNelson https://www.roblox.com/library/143613186/Model-Resize-Plugin-2-1-DRAG-TO-RESIZE I only use this plugin if the regular roblox resize won't make the model as small as it needs to be.
Material Flip by stravant https://www.roblox.com/library/166951203/Stravant-MaterialFlip I mainly use this on "Wood Plank" material or a custom texture to make it face a certain direction, usually in a "feel good" direction to help guide the player in a walking direction I'd like them to go.
Free Model Virus Remover by JetCrusherTorpedo https://www.roblox.com/library/142273772/Ro-Defender-Plugin-v8-7 I haven't used this in a long time, but it was helpful in my earlier days when I didn't know how to read all of a free model's contents to know if it has something malicious or not.
Creating Tools: Tool Grip Editor by EgoMoose https://www.roblox.com/library/153352998/Tool-grip-editor Edits the positioning/orientation of how the Tool looks in the RightHand
Born2Weld by Born2Script https://www.roblox.com/library/342043284/Live-Weld-Plugin-Born2Weld-Updated tools can't be anchored otherwise you wouldn't be able to pick them up and walk with them, so in order for a multiple part tool to stick together/not fall apart, they must be welded, one way is using this plugin