RFI Services,  Mike Martin, K3RFI

-- RFI Services is a company located in Maryland that specializes in locating RFI, primarily from powerlines. Several powerline noise recordings, plus links to some interesting articles.

SDR RFI Snapshots by NK7Z

--The above link provides a collection of various RFI sources as captured using SDR hardware.  These spectrograms can assist in the identification of RFI sources.
 

http://www.kb9ukd.com/digital/

-- An extensive collection of audio files of many digital modes. Many were recorded in the HF bands an most others are found on VHF/UHF frequencies.

http://www.milspec.ca/modems/modems.html

-- A collection of sounds of commercial and military digital modes from Richard Lacroix's (VE3UVX) Military Communications Website. Also includes some spectral displays of the files.

http://www.nzart.org.nz/nzart/ms/index.html

-- From the New Zealand Amateur Radio Transmitters (NZART), this small but carefully selected collection of audio samples represents some of the most common intruders, both analog and digital, in our Amateur bands.

http://www.iarums-r1.org/iarums/sound/main.html

-- IARU Region 1 Monitoring System Soundfiles - consists of several samples of military digital communications, several HF Over-the-Horizon radar systems and some others of interest.

An article by W4LKE on how he uses a DMM with serial communication capability to record occurrences of powerline noise to provide hard evidence to his local power company of interference he is experiencing.

Click this link to view the article www.arrl.org/noise-logging