DesignCenter

Organizes access to drawings, blocks, hatches, and other drawing content.

View tab > Palettes panel > DesignCenter

C:\Program Files\Autodesk\AutoCAD 2016\Sample\en-us\DesignCenter

You can toggle on/off the Folder List and Preview
You can drag & drop the blocks into the current drawing

Double-click on the block in the content area
Insert dialog box pop-up let you specify the scale and rotation
If the block created in inches, it will convert to metric when insert into a metric drawing.

AUTODESK* SEEK let you download free blocks from Internet