I subscribe to a number of different mailing lists related to radio topics. Something that has intrigued me for quite some time is VLF or Very Low Frequency radio, especially so-called Part 15 or “lowfer” operation. There has lately been some overlap with my interest in software defined radio. Some people have recognized that there is a family of fairly inexpensive video capture cards based upon the Brooktree 878 chipset which can be retargeted to do high A/D conversion of raw signals. These cards are quite inexpensive (the last one I got was about $40 or so), so they might make a reasonable platform to create a SDR platform.

Check out BT878 Audio Capture Trials for some information about what is possible.