Japanese blogger and RTL-SDR experimenter ttreftech has had an ADS-B front end kit (In Japanese, use Google Translate) consisting of a low noise amplifier (LNA) and SAW filter available for sale in Japan for a few months now. The LNA helps to push weak signals through the coax feed line and the SAW filter is a bandpass filter that helps to remove interference outside of the 1090 MHz ADS-B region. If you are interested in building your own version, ttrftech has also posted a schematic. Another recent post about the front-end can be found here.
Another Japanese blogger, “pup” has posted about his results with the ADS-B front end kit (Also in Japanese, use Google Translate). His results show that the front end does significantly improve ADS-B reception. The image below shows an ADS-B signal with the front end turned off (top) and with it turned on (bottom). Pup has also posted a video showing the kit and its performance on HDSDR.