Reddit user BigReid has posted on the Reddit rtl-sdr forums his homemade bandpass filter with a pass range of 1-1.2GHz for improving adsb reception. A bandpass filter blocks any interference from signal frequencies outside of the pass range. This might be useful for adsb if you experience a lot of interference from out of band signals at 1090MHz.
His filter is a hairpin filter, which can easily be etched onto a PCB board. You can find the PCB schematic files linked on the post.
Blogger John Wiseman has managed to get ADS-B decoding using an rtl-sdr stick working on his AR.Drone quadrocopter. The AR.Drones run on the Linux operating system, so he was able to compile and install the Linux ADS-B decoder dump1090 on his drone.
Although the reception was hampered by RF interference from the drones electric motors, it was still able to pick up a number of ADS-B signals.
The RTL-SDR can be used as a super cheap real time air radar. Modern planes use something called an ADS-B (Automatic Dependent Surveillance-Broadcast) Mode-S transponder, which periodically broadcasts location and altitude information to air traffic controllers. The RTL-SDR can be used to listen to these ADS-B signals, which can then be used to create your very own home aircraft radar system. Compared to dedicated commercial ADS-B receivers which can go for between $200 - $1000, the $20 RTL-SDR is very attractive for the hobbyist in terms of price. However, note that the RTL-SDR probably shouldn't be used for ADS-B navigation in a real aircraft for safety reasons.
ADS-B broadcasts at a frequency of 1090 MHz. It has been discovered by the RTL-SDR community, that the RTL-SDR with R820T tuner has the best sensitivity at this frequency. The E4000 and other tuners perform poorly in comparison. So it is recommended that you obtain an R820T tuner if you want to set up ADS-B decoding with the RTL-SDR. Recently there has also been talk about the R820T2 tuner, which seems to have slightly better performance too. See the Buy RTL-SDR dongles page for more information on where to purchase.
We also now note that recently new higher end SDR's like the $199 Airspy have developed very good ADS-B receivers that are several times more sensitive that the RTL-SDR.
Examples of RTL-SDR used as an ADS-B air radar
In this video, YouTube user Superphish shows a timelapse of air traffic over New Zealand using RTL-SDR, ADSB# and Virtual Radar Server.
ADSB Virtual Air RADAR with RTL SDR (RTL2832), ADSB Sharp and Virtual Radar Server