For a close connection:
* As close as two peas in a pod.
* Like two sides of the same coin.
* Like a lock and key, they fit perfectly together.
For a weaker connection:
* As connected as a string of beads.
* Like two ships passing in the night.
* Like a shadow that follows you, but never catches up.
For a negative association:
* As connected as a bad smell to a garbage dump.
* Like a stain that won't come out.
* Like a thorn in your side.
For a specific type of association:
* As connected as a brand name to its products.
* Like a cause and its effect.
* As intertwined as the roots of a tree.
The best simile for "associated" depends on the context and the specific relationship you're trying to describe.