SD (software-defined) radio receivers use a bare minimum of hardware, relying instead on their software capabilities. This SDR project demonstrates what’s achievable, in this case a multi-purpose receiver covering all bands from 150 kHz to 30 MHz. It’s been optimised for receiving DRM and AM broadcasts but is also suitable for listening in to the world of amateur transmissions.

www.nti-online.de/diraboxsdr.htm
www.sdradio.org
sourceforge.net/projects/drm
www.g8jcf.dyndns.org

• OpenBTS is a software-based GSM access point, allowing standard GSM-compatible mobile phones to make telephone calls without using existing telecommunication providers' networks. OpenBTS is notable for being the first free software implementation of the industry-standard GSM protocol stack.
• The GSM Software Project: We want to bring together all the folks that are interested in building a gsm receiver.

http://www.qsl.net/py4zbz/DSP/softw.htm "My first SDR is a software demodulator SSB (USB and LSB) with AGC, to be used in conjunction with a hardware SDR such as SDRZero. The I and Q signals produced by SDRZero are applied at the entrance of the stereo sound card, I being in the right channel and Q in the left. Comentarei, com ilustra ões feitas com o próprio BasicDSP, o programa exemplo. Comment, with ilustraões made with the very BasicDSP the program example."