General Worthlessness:
* Not worth a dime: Meaning it's completely worthless.
* Not worth the paper it's printed on: Used for documents or contracts that have no legal or practical value.
* Not worth a hill of beans: Emphasizing complete lack of value.
* Not worth its weight in salt: Meaning it has no practical use.
* A piece of junk: Simple and straightforward, implying it's broken or useless.
Emphasizing Futility:
* A waste of time: Suggesting it's pointless and serves no purpose.
* A dead weight: Meaning it's useless and burdensome.
* A white elephant: Something that is expensive to maintain but has no real value.
More Specific Idioms:
* A dog's breakfast: Used for something messy, disorganized, and useless.
* A lemon: Used for a faulty or defective product, especially a car.
* A dud: Used for something that doesn't work or perform as expected.
The best idiom to use will depend on the specific context and the specific type of worthlessness you want to describe.