Say you have a component based entity system and your system is kind of normal:
Where an Entity is a ComponentContainer and an AttributeContainer.
Components are things that use attributes (The entity holds the attributes so that two components can say...access the Mesh attribute or health...) and can add\get attributes to an Entity.
Attributes are pretty obvious.
Now, lets say you have your Renderable3D component, and when it gets added to an Entity (Or created, you can't have a component without an Entity...at least in my system you can't) you need it to also get added to the Renderer so it can be rendered...
How would you do this? Make the Renderer a singleton? (OnEntityAdded or Initialise on the component could then just add itself...)