Shallots, scallions, leeks, onions (red, white, yellow, sweet, pearl, cippoline) etc.....
All of these have similar flavor profiles, but also have subtle differences. While we all certainly produce fine dishes by substituting whichever we have on hand, the results will not be exactly the same. For example, the robustness of some onions would overpower the delicate flavors of the tarragon and butter in a bearnaise, while the more delicate flavor of a shallot complements it nicely. A shallot would get lost in a hearty chili or stew, while an onion is critical to its flavor.
If you're subbing strong onions for shallots, I'd use a little less than what's called for.