State machines come to mind: a transition is just a function call. Unfortunately that's a general tail call, not always a recursive one, so no love from this library, and that's where "proper" TCO wins (or trampolines if $your_language lacks TCO)

Also it wouldn't help with Fibonacci, since while it's recursive, it's not tail-recursive (yes, it can be written that way, but I'm talking about the idiomatic naive definition).