22 (unless I lost track of the ordinality)
ADS-B, ADS-R and FIS-B and UAT
ADS-B (I have working in gnuradio) by 2020 all aircraft need to be equipped with ADS-B Out. This broadcasts lots of info depending on the configuration and other things. But at least you get ICAO Mode-S transponder number which is unique to each aircraft. Can also get speed/heading/lat/long/call sign and more. 1090MHz
ADS-R Are rebroadcasts from air-to-air and is somewhat new, this is interesting because it is essentially creating a mesh-network in the sky. This might also help eliminate bogies. i.e. bad actors tx ADS-B frames to make ghost planes appear.
Also at 1090MHz (I think)
FIS-B Is the free data that the FAA gives to planes equipped with ADS-B Out/In and that are also compliant hardware installation. Like a reward I guess. They give you wx radar info and other wx info as well as air traffic info. But I think this requires line-of-sight to the airfield.
UAT at 978MHz is allowed for aircraft flying under 18,000 ft. It is an alternative to ADS-B and sends data in a format that differs from ADS-B but most commerical aircraft can rx this and then tx it onto ADS-B via ADS-R. UAT is a lower cost way to
comply with the FARs for 2020 deadline (which may get extended). Apparently the FAA thinks that all aircraft owners have deep pockets but it's not cheap to install
There's tons of stuff on github and there is even a VHDL decoder for ADS-B for bladerf.
dump1090 and dump978 are good search terms.