Modern aircraft have a number of systems to their disposal to enhance flight safety. The common term is aircraft collision avoidance systems.
These range from radar, TCAS, portable CAS, terrain awareness system (TAWS), ground proximity warning system (GPWS), synthetic vision in EFIS and the obstacle collision avoidance system.
TCAS or Traffic Collision Avoidance System interrogate other transponders and process these replies from other aircraft enabling them to show these on the PFD, MFD displays or a special IVSI indicator.
Some even issue aural warnings to the pilot helping him to avoid a potential midair collision or even flight into terrain.
For the experimental homebuilt aircraft pilot these are usually quite expensive systems, except for the portable CAS and the good old see and be seen system (which has its flaws). New ADS-B In solutions offers a receiver with connection to a mobile device as weather/ traffic display.
Mode S transponders even exchange data with TCAS so that aircraft equiped with these systems know where you are and are able to remain at a safe distance from you and other traffic.
These systems can only operate 100% reliable if all aircraft are equipped with at least a Mode C SSR transponder. The TCAS system will interrogate aircraft transponders via 1030 MHz and receive replies on 1090 MHz. With this information it is aware of other, possibly conflicting, traffic in the vicinity.
And by constantly interrogating, the TCAS system can see in which direction traffic is moving (speed and time) and if it might be coming too close. If so, then the pilot receives a visual or aural warning of that traffic with an advisory.
And as more and more aircraft are being equipped with transponders, mandatory or not, this system works quite well.
A number of manufacturers sell portable passive traffic detectors based on receiving transponder transmissions by other aircraft. It is basically a listening device with indicators on the frontpanel displaying signal strength, translated into range. These devices detect traffic within a 5 to 7 nm radius and relate signal strength to distance in a circle.
When traffic gets really close (within 3 nm or so) the indications usually will change to be more pronounced. Mode S transmissions can sometimes be displayed by a different color indicator.
More sophisticated models use the mode C altitude transmission of other aircraft to compare that with their own mode C to indicate the relative altitude of the target.
Integration with the aircraft intercom system is also possible and advisable. Note that some models are battery powered where others need a connection to the aircraft avionics bus (which must be fused).
If you even had a near miss or if you suddenly saw an aircraft too close for comfort, then by all means invest in at least a system which gives warnings of traffic closeby. It will save your life one day.
Those of you flying behind Garmin LCD screens will find the above systems built-in the EFIS system for easy use and top notch integration.
Automatic Dependent Surveillance-Broadcast (or ADS-B) is part of the US Next Generation Air Transportation System (NextGen). It requires no input from you, but needs integration with other aircraft systems like a GPS receiver for its position and a transponder for transmissions. With an external display (either standalone or an EFIS) traffic can be shown to the pilot. There are two options for aircraft/pilots: ADS-B Out, which transmits only and a ADS-B In as a receiving solution. Combined versions are also possible but are more expensive.
There are several portable/ mobile systems in use today: FLARM and PilotAware (which uses a Raspberry PI and a GPS, RTL and Wifi dongle) to name but a few. Garmin offers a portable solution which also includes subscription free weather including NEXRAD radar, METARs, TAFs, TFRs, AIRMETs, SIGMETs and NOTAMs. Appareo offers all three ADS-B options for a reasonable price.
Uavionix (www.uavionix.com) sells a number of ADS-B in and out (transponder) solutions for the experimental and light sport market. They even carry mini ADS-B units for drones!
Note: ADS-B In works only when a ADS-B Out is triggered by an ground facility in the area, only then you will see traffic on your ADS-B In solution.