Are there any open / unrestricted radio frequencies that are free for any use?

There's the ISM bands that you can use for whatever you want, as long as you stay withing certain restrictions, mainly power level.

The 2.4Ghz band is one such ISM band, which is why there is so much traffic on it (WiFi, Bluetooth, Analog video cameras, ZigBee). For that band, I believe that the power limit is 1 milliwatt for continuous transmission, and 10 milliwatt for low duty-cycle transmission. No license or certification is required, but the FCC would come after you if you exceed the limits.

Wikipedia has a list of ISM frequencies: https://en.wikipedia.org/wiki/ISM_band

The two bands listed as "Amateur" are usable, but you need to know what you are doing.

Sorry, no, at least not in the US. The FCC has allocated everything between 9 kHz and 275 GHz.

No, not for normal interpretations of "radio". Think about it. If there were such a frequency range, lots of people would be abusing it, thereby making it unusable. That's why we have a central authority that allocates space. Doing so is in everyone's interest, even if not in individual interests.

It's a lot like the rules we have for driving on a specific side of the road. If there is a traffic jam one way and little traffic the other, you as one person would be better off driving on the other side of the road. However, when everyone does this, as they would if there weren't any rules, we'd have a dangerous mess, and everyone would be worse off.

You can use much shorter wavelength, like IR and visible light, pretty much any way you want for communication. Even then, you can't go lighting your neighbor's house on fire with a stron IR beam or point a laser in the eyes of airplane pilots.