GNU Radio is a very simple idea, somewhat akin to the idea of winmodems: take the job performed by special-purpose hardware and handle it in software. The hardware GNU Radio wishes to aid is radio hardware. What this means is that instead of needing special hardware to, say, perform FM demodulation or output a sine wave to a computer speaker or show an FFT display of analog DC input, you can use simple and cheap hardware and let your computer's CPU do the rest. Thanks to the miraculous workings of Moore's Law, it is possible for someone without huge amounts of money to process digital signals however they want (or however they can program).

This could also lead to the manufacture of very cheap wireless cards. Such a card could function as an IEEE 802.11b card, or use Bluetooth, or whatever other wireless protocols people can come up with, without having to upgrade hardware.

Of course, this could lead to a horrible mess of software incompatibility and drivers not being available for any operating systems except Windows, but the GNU Radio developers are hoping to avoid this hazard by making the source code available to anyone under GPL.

There is another issue that makes winmodems evil which GNU Radio mostly sidesteps. By offloading the tasks performed by special hardware in real modems to the CPU, winmodems can seriously slow down your computer for a trivial task that could have been easily performed by a couple cheap chips in the modem. For the tasks that GNU Radio performs, the special-purpose hardware is anything but cheap or trivial. It is assumed that anyone who wants to, say, broadcast their DivX collection over the radio on some channel that isn't being used will not try to run GNU Radio in the background while doing other thing like playing Quake. The benefits outweigh the evil, if only people don't assume guilt by association with winmodems.