With VBA class modules we can create our own reusable objects.
A good introduction into VBA classes you can find at Chip Pearson's website.
During my 30 years of Excel practice I have encountered some very useful classes which I like to present here:
SystemState - This one I am using in almost all of my serious VBA applications to speed up my code.
Logging - This class is very useful if you like to document each execution (run) of your VBA code.