Basic SDR's but lots of FUN 
I bought a RTL2832U receiver a year ago for $20 and was excited to get it working but just couldn't get it going. I got frustrated with it so I shelved it for a month until I decided to try again and eventually figured things out. I bought a second dongle, the R820T for $12 and have thoroughly enjoyed it with HDSDR (my preferred software on ancient computer). Recently a dear friend gave me his FUNcube Dongle Pro, which turns out to merely be a standard RTL2832U receiver with a pre-adjusted dll for better listening.
I also tried ADS-B but like some others, couldn't get it to work but admittedly now that I've become more proficient with configuring these little recievers I'm gonna give it another go and I'm sure I'll get them working. I sometimes use it to monitor ACARS traffic, although this is a far less endeavor than ADS-B monitoring which is why I really want to get it working! Strangely, I find that I use these el-cheapo receivers far more than my more expensive radio's (Icom R8500, R75, Kenwood TS-2000, TM-D710, etc), as I only listen anymore. I recently added the Ham-It-Up upconverter with their new extruded aluminum cases they now sell and it works wonderfully with my R820T. In fact, I'm listening to 15 meters as I write this and I personally find these cheap SDR's much more fun to use than traditional style radio's. As a result I'm going to sell my Icom R8500, R75 and Kenwood TM-D710 because I haven't used them in months. A year ago I never would have guessed I would be this impressed with these simple little receivers but they are a lot of fun to use and work great. I really enjoy being able to "see" a more complete picture of the band I'm listening to, and it's quite interesting once you learn to identify various RF signatures in the waterfall display. I also use various software to decode numerous types of digital traffic. I enjoy APRS and traditional packet radio and we still have a local packet group with a great network. I still have a real TNC and a dedicated 2-meter transceiver to "keyboard" with the local packet group but I use the R820T far more to monitor digital activity than my old setup.
Once you figure things out these little SDR's are a lot of fun and provided you have decent antenna(s) you can listen to the world with almost no investment in your receiver gear, which traditionally has always been the most expensive part of the shack
