Not sure if these count as plug-ins, but if anyone wants to convert them, they'd probably be very useful.
- Camera Cutscene-creation plug-in:
http://www.roblox.com/x-item?id=76781595 - Local and Remote Error Monitoring System and Place Console:
http://www.roblox.com/x-item?id=65774258 - Random Utilities:
http://wiki.roblox.com/index.php/User:HotThoth/Metascript_Guide. Note: The UI is terrible; the functions might be helpful (especially the workspace search ones). Don't remember what's there, but feel free to scavenge.
Got a couple broken / unfinished ones which would be very cool if I ever finish them, esp. if someone wants to do the GUI for them, lol (model/humanoid resizing, Motor6D-creation/adjustment, and a "Perma-hinge" one are the three that come to mind). The last one allows you to create hinges like those on cars, but they don't use SurfaceWelds, so you can have vehicles where the wheels are apart from the body and which don't fall apart when cloned. It's done except for a GUI.