Apps
MorseAll — keyboard input via morse code on a mouse. Very mature and complete in features.
DMOZ: Browsers, Accessibility
AT&T Lab's Automated Vehicles
GPSS
Voice Browsers (1998)
Door-to-door Copilot (GPS software)
TTS Konquerer
Car MP3 player, implements various ways to control the mp3 player

Integrated Car Apps
Delta Dash — Imprezza-specific virtual dash, with lots of ECU-specific good stuff
HKS CAMP — virtual dash of sorts
Extreme GEEZ — accelerometer monitor, draws nifty autoX maps
other accelerometers here, in my product_searches directory
UTEC dashboard, for use with the UTEC piggyback ECU, as well as any of the other ECUs with datalogging capability

Linux Apps / libraries
SpeechWorks (a VoiceXML interpretter)

Use Cases
Navigation
MP3 (passive entertainment)
Web stuff (traffic, general surfing...)
TV
Radio
DVD / DivX
Games (eg. MAME)

Standards
VoiceXML spec
Aural CSS. With its cue-before, cue-after, and play-during css attributes, it's absolutely ideally matched to what I was thinking of doing. Apps which support this include emacspeak, websound, pwWebSpeak. Even if I ultimately decide that a suitable browser isn't available, at least an excellent language is available that I can base my development off of. If Emacspeak looks good, perhaps the Emacs::Lisp module could be used from perl to control Emacs, and thus Emacspeak.

Very high level design
BBN Position Paper on Conversational Web Access

Indexes
Linux Accessibility HOWTO