What app is so useful, you can’t believe it’s free?

For me, Reaper without a doubt. I've been doing audio work for the better part of a decade now it's become my main DAW of choice for the last 5 years. Technically a license costs you $60, but every feature is available in the "trial" version that never expires. SWS script support and the insane amount community skins/extensions makes it well worth the $60 if you plan on buying. I've replaced many ProTools with Reaper setups.....