Modern aircraft have a number of systems to their disposal to enhance flight safety. The common term you will hear is aircraft collision avoidance systems. These systems 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 nowadays offers a receiver with connection to a mobile device as weather/ traffic display.
Automatic Dependent Surveillance-Broadcast (or ADS-B) is part of the US Next Generation Air Transportation System (NextGen) and mandatory in the USA as of 2020. 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 an ADS-B In as a receiving solution. Combined versions are also possible but are more expensive.
The situation in Europe is somewhat difficult as you can imagine. So many countries so many opinions and EASA is hard at work with their SERA, Standardized European Rules of the Air. Meanwhile AOPA France held a presentation about ADS-B and if it was important to GA. In a nutshell: it does not matter, we need to equip our aircraft sooner or later. Read more in the next link: AOPA France ADS-B
There are several portable/ mobile systems in use today: Stratux, FLARM, Merlin and PilotAware (the last two use a Raspberry PI and a GPS, RTL and Wifi dongles) to name but a few. These require your to assemble all parts and together with two antennas and an USB power bank you will be ready to go for the least amount of money spent. Although some can be bought pre-built, either through Amazon or direct.
Garmin offers a portable solution (GDL 50 series) which also includes subscription free weather including NEXRAD radar, METARs, TAFs, TFRs, AIRMETs, SIGMETs and NOTAMs. The GDL 52 combines ADS-B weather with XM for your portable device via Wifi. Appareo offers all three ADS-B options for a reasonable price.
This company (www.appareo.com/aviation) sells the Stratus series of ADS-B solutions. The latest version is the Stratus 3 with WAAS GPS, AHRS, Flight Data Recorder, pressure sensor and Wifi.
Uavionix (www.uavionix.com) sells a number of ADS-B in and out (transponder) solutions for the experimental and light sport market, the Scout and Sentry (ForeFlight) being the latest and greatest. They even carry mini ADS-B units for drones!
Levil Aviation (levilaviation.com) sells a combined wing light beacon system. The unit looks like a mini fin which has a pressure sensor, transponder receiver, GPS receiver and an ADS-B Out system. It mounts on the belly of the aircraft and needs a separate GPS antenna, listens to your transponder code and sends out the ADS-B data. They even have an under the wing mounted unit, BOM.
Dynon Avionics (www.dynonavionics.com) sells a portable ADS-B solution in the form of a small device capable running a weekend long and its called the DRX. It sits between the full blown and the do it your self Raspberry PI solutions. The DRX has a dual band ADS-B receiver and supports the GDL-90 protocol via Wifi to numerous compatible applications as Foreflight, FlyQ, SkyDemon (needs a barometric indication) and more.
ForeFlight (foreflight.com/support/sentry/) also sells a portable ADS-B solution: its called the Sentry with similar capabilities but its only for the foreflight app. It does however have an AHRS backup, CO monitor, pressure sensor and 12 hour battery.
Note: ADS-B In works only when an ADS-B Out is triggered by an ground facility in the area or your own ADS-B Out transponder, only then you will see traffic on your ADS-B In solution.