Radio Signal Identification

•  Signal Identification Guide 
         http://www.sigidwiki.com
• In general (with samples)
         http://www.rtl-sdr.com/signal-identification-guide/
  
• Digital modes (with samples)
  
  • http://hfradio.org.uk/html/digital_modes.html 
  • http://www.kb9ukd.com/digital/

• Practical Signal Detection And Classification In GNU Radio (PDF)
  http://www.cs.umd.edu/~clancy/docs/sdr07-detect.pdf