Here is a simple adjustable brightness flashlight circuit. This is the main thing I want depletion mosfets for!

Apart from being simple, the circuit has a few practical features.

* Continuous brightness adjustment flashlights are quite rare.

* Flicker-free. Important for video/photography.

* This circuit allows to adjust the brightness in extremely wide dynamic range, from totally off to barely visible in complete darkness to full power the LED can handle.

* The input current required by mosfet gate is negligible, which makes it possible to put very large resistance potentiometer. Dodgy viper contact isn't going to cause any issues.

* There is no need for on/off switch - the adjustment pot does it. The off current can be made as low as desirable by increasing potentiometer resistance, and can possibly be even made negligible compared to self-discharge due to cell protection circuitry.

The main disadvantage of this circuit is that it's relatively inefficient, although I think this is easily compensated for by lowering brightness as needed.

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more potential applications:

low or negative voltage drop voltage follower
-> LED brightness control (as just shown)

-> LDO regulators
-> amplifier output stage (rail-to-rail with no bootstrap)

inrush current limiter
-> DC
-> AC