I'm looking for a word meaning a thing that essential and indispensable, but with strong connotations of being functional and useful. This thing is essential because it is useful.
The closest I've been able to find is necessity, but that doesn't seem to highlight the functional nature of the thing in question. On the other hand, a term like utilitarian doesn't imply indispensable.
Example Sentence: Stack Exchange has become X, it's hard to imagine living without something so useful.