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.