An underground alternative to the wired Internet
DSP Resources
The future of Amateur Radio will involve Digital Signal Processors.  They will be used for noise cancellation, filtering, and symbol generation/regeneration.  As I begin my research into these 
realms, frequently check information will be stored here.

PC Soundcard DSP chip's

Most people don't realize it, but there is a powerful DSP chip in most systems bought on the market today.  They are emulated using the hosts microprocessor (486 and greater.).  The linux operating system includes programmable DSP support via a /dev/dsp device.  Save those old Soundblaster Pro/16's, they are the easiest to use for packet radio.

Drivers for many soundcards are available in the current linux kernel, however, Plug-and-Play cards are not well supported.  The Open Sound System (4-front) has drivers that will support PnP cards, but is not free.  However, a Czech effort has surpassed the functionality of the OSS outfit, the Advanced Linux Sound Architecture (ALSA) is moving to the forefront of sound accessablility for Linux.

The linux Sound HOW-TO will shed some light on what soundcards are currently supported and how to configure your soundcard under linux.

Discrete DSP chip's

Aside from Soundcards, there are various standalone DSP's which can be purchased at low cost.  To become more familiar with DSP and its associated black magic, I suggest you check 
out the DSP FAQ.