Here are a few

qslideanswer - http://talk.maemo.org/showthread.php?t=93257 fast portrait only way I know to be sure to pop up before you miss the call, occasional caller ID problems, no source as of June 3, 2014

fldigi - http://talk.maemo.org/showthread.php?t=82853 amateur radio decoding app useful with line-in from a radio

RTL-SDR - http://talk.maemo.org/showthread.php?t=91182 with a cheap USB dongle and adapter you can no do software defined radio with a N900, almost limitless applications for about $20 in hardware.

Beats Audio Driver - http://talk.maemo.org/showthread.php?t=91010 not really a driver but some scripts which tweak sound settings(use with caution)