This a general question really, not sure if this is the place for it (it might be deleted as quite general) so please don't heckle (I am just curious).
I have been reading up on WebWorkers API and had a thought.
WebWorkers can be limited to using only small amounts of processing power for each machine/user. This could be tailored to not affect user experience and might only slighly affect browser performance (if at all).
My question is, could they theoretically be used to turn a website/application into a highly distributed supercomputer?
Is it more of an ethical question as IF it could be done, is it wrong if the user is not aware?
It's possible, unethical and likely illegal.
It is certainly possible to do. In fact you don't even need to use web workers to do it. It is probably unethical to do if the user is not aware but it may not actually be degrading to the user experience or even noticable. It may even be illegal and you should get some legal advice.
For example, if you have an aplication where users are aware that they help folding proteins while playing your game or something like that then it may be a great application. If, on the other hand, you want to mine bitcoins using the processing power and electricity of your unsuspecting visitors then you are asking for trouble.