Symphony.

I've always managed to get around the problem before, but is there an extension that handles the ability to show a field, only if another field meets X,Y,Z criteria?

For example if a checkbox is ticked, show a textarea to provide details (which would be optional and otherwise hidden).

Of course the above example would be trivial to achieve with a single line of javascript, but is there an extension solution out there for dealing with such scenarios (and likely more complex ones) that I've managed to miss?

There's an outdated extension ( ;-) going into that direction, but not exactly what you're looking for, I guess.

I guess it would be hard for an extension like this to be flexible enough for multiple use-cases. The single line of JS/jQuery you mention always worked fine for me.