Today's technology allows us to build radio receivers that sample radio signals and process them on a PC or an embedded system. Similarly, transmitters can be built that generate the RF signal digitally, then convert it to analog. Software Defined Radio (SDR) refers to the technologies that make these exciting things possible.
This site has been created to popularize SDR technology. With the help of a cheap USB DVB-T dongle you can get into SDR quite easily. It allows you to listen to the radio signals around you, but it also lets you to learn about DSP and code your own receiver.
On sdr.hu, you can find SDR receivers that amateur radio operators shared, so you can listen to radio signals without even having to buy any SDR hardware! In fact, amateur radio is a great thing and also lets you experiment with transmitting on the air, by using various frequency bands and modulations.