With the Silver Key artifact, what effects can I spend 1 less clue?
After considering the links in this thread, I've concluded that the Silver Key artifact can be used anywhere you might spend a clue including re-rolls. The single situation I would say it cannot be used is where the wording says after resolving a research encounter, the investigator can spend a clue gained from that encounter to etc such as in the Azathoth Occult Research mystery card.

The whole purpose of this wording seems to be to imply that the investigator should have succeeded in gaining that specific clue in order to spend a clue, and not allow one to circumvent it (i.e. via the Astronomer ability) to supply a clue.

Apart from that case, it's an artifact, so should be viewed as all manner of awesome. A 1 clue per round boon in every non-specific clue interpretation sounds fair game to me.