Types of Trailer
These themselves come in many different designs and can also be used as general cargo carriers:
Trailer only such as the Burley Bee.
Stroller and bike trailer such as the Thule Coaster XT
Multi sport trailer like the Thule Cross
Good for putting your own size box or bag on top or for fixing oversize items on. Carry Freedom are a high quality manufacturer of this type of design.
Bikes at Work produce extra heavy duty and extra large flatbeds for really big loads:
Special Purpose Trailers
As the name suggests, these are designed for a specific purpose.
The Radical Chubby trailer is designed to take a folded Brompton: