MyBox is a huge collection of useful stuff made by me and with a few peaces from other people over the years.
Wiki on Attributes, Tools and Features, Types. Extensions docs is lacking for now 
Some of my favorite stuff:
ConditionalFieldAttribute — display one field based on some other fields
DisplayInspectorAttribute — one inspector inside of another
MustBeAssignedAttribute — assert values automatically
SeparatorAttribute 
AssetPressetPreprocessor
Apply asset presets on asset import, based on path or name of the asset
Auto save on playmode
Optional and OptionalMinMax types
And also tons of really helpful extension methods, like
myVector.SetY(0).ClampX(-5, 5);
myImage.SetAlpha(0);
waypoints.GetRandom();
and many more
