Firmware refers to embedded software which controls electronic devices. Well-defined boundaries between firmware and software do not exist, as both terms cover some of the same code. Typically, the term firmware deals with low-level operations in a device, without which the device would be completely non-functional... (read more on Wikipedia).
Devices/Drivers Firmware
Many devices require a firmware to operate. Historically, firmware were built-into the device's ROM or Flash memory, but more and more often, the firmware has to be loaded into the device by the driver during the device initialization.
Some of these firmware are free and open-source, and some of them are non-free (which means that you need to add the non-free and contrib components to /etc/apt/sources.list, see sources.list(5), or apt-howto).).
Location of the firmwares
The firmware are stored in the following places (see udev's /lib/udev/hotplug.functions)
/lib/firmware/ - Firmware provided by a package, valid for all kernels - up-to DebianLenny.
/usr/local/lib/firmware - Location for manually installed firmware.
/usr/lib/hotplug/firmware - Firmware provided by a package, valid for all kernels
/lib/firmware/`uname -r`/lib/firmware - Firmware provided by a package, specific for a kernel - new in DebianSqueeze??.
List of firmware in linux kernel
To find which package provide a given firmware file, you can use this search page:
http://www.debian.org/distrib/packages#search_contents
Here's a list of Linux modules that requires a firmware to operate, generated automatically from Lenny's linux 2.6.26-686 kernel.
Note: this list may not be complete (because the firmware name can't be extracted reliably).
microcode.ko
Intel CPU (IA-32) Microcode Update Driver
bcm203x.ko
Broadcom Blutonium firmware driver ver 1.1
bfusb.ko
BlueFRITZ! USB driver ver 1.1
bt3c_cs.ko
Bluetooth driver for the 3Com Bluetooth PCMCIA card
ip2.ko
Computone ?IntelliPort Plus Driver
ip2.ko
Computone ?IntelliPort Plus Driver
dell_rbu.ko
Driver for updating BIOS image on DELL systems
tuner-xc2028.ko
Xceive xc2028/xc3028 tuner driver
xc5000.ko
Xceive xc5000 silicon tuner driver
b2c2-flexcop.ko
?B2C2 FlexcopII/II(b)/III digital TV receiver chip
dvb-bt8xx.ko
Bt8xx based DVB adapter driver
dvb-usb-gp8psk.ko
Driver for Genpix 8psk-to-USB2 DVB-S
dvb-usb.ko
A library module containing commonly used USB and DVB function USB DVB devices
dvb-usb-opera.ko
Driver for Opera1 DVB-S device
bcm3510.ko
Broadcom BCM3510 ATSC (8VSB/16VSB & ITU J83 AnnexB FEC QAM64/256) demodulator driver
nxt200x.ko
NXT200X (ATSC 8VSB & ITU-T J.83 AnnexB 64/256 QAM) Demodulator Driver
or51132.ko
OR51132 ATSC [pcHDTV HD-3000] (8VSB & ITU J83 AnnexB FEC QAM64/256) Demodulator Driver
or51211.ko
Oren OR51211 VSB [pcHDTV HD-2000] Demodulator Driver
sp8870.ko
Spase SP8870 DVB-T Demodulator driver
sp887x.ko
Spase sp887x DVB-T demodulator driver
tda10048.ko
NXP TDA10048HN DVB-T Demodulator driver
tda1004x.ko
Philips TDA10045H & TDA10046H DVB-T Demodulator
pluto2.ko
Pluto2 driver
budget-av.ko
driver for the SAA7146 based so-called budget PCI DVB w/ analog input and CI-module (e.g. the KNC cards)
budget-ci.ko
driver for the SAA7146 based so-called budget PCI DVB cards w/ CI-module produced by Siemens, Technotrend, Hauppauge
dvb-ttpci.ko
driver for the SAA7146 based AV110 PCI DVB cards by Siemens, Technotrend, Hauppauge
ttusb_dec.ko
TechnoTrend/Hauppauge DEC USB
bttv.ko
bttv - v4l/v4l2 driver module for bt848/878 based cards
cx18.ko
CX23418 driver
cx23885.ko
Driver for cx23885 based TV cards
cx23885.ko
v4l2 driver module for cx23885 based TV cards
cx25840.ko
Conexant CX25840 audio/video decoder driver
cx88-blackbird.ko
driver for cx2388x/cx23416 based mpeg encoder cards
ivtv.ko
CX23415/CX23416 driver
pvrusb2.ko
Hauppauge WinTV-PVR-USB2 MPEG2 Encoder/Tuner
acenic.ko
AceNIC/3C985/GA620 Gigabit Ethernet driver
bnx2.ko
Broadcom ?NetXtreme II BCM5706/5708 Driver
cxgb3.ko
Chelsio T3 Network Driver
irda-usb.ko
IrDA-USB Dongle Driver
myri10ge.ko
Myricom 10G driver (10GbE)
at76_usb.ko
Atmel at76x USB Wireless LAN Driver
atmel.ko
Support for Atmel at76c50x 802.11 wireless ethernet cards.
b43.ko
Broadcom B43 wireless driver
b43legacy.ko
Broadcom B43legacy wireless driver
ipw2100.ko
Intel(R) PRO/Wireless 2100 Network Driver
ipw2200.ko
Intel(R) PRO/Wireless 2200/2915 Network Driver
iwl3945.ko
Intel(R) PRO/Wireless 3945ABG/BG Network Connection driver for Linux
iwl4965.ko
Intel(R) Wireless WiFi Link 4965AGN driver for Linux
libertas_cs.ko
Driver for Marvell 83xx compact flash WLAN cards
libertas_sdio.ko
Libertas SDIO WLAN Driver
usb8xxx.ko
8388 USB WLAN Driver
p54pci.ko
Prism54 PCI wireless driver
p54usb.ko
Prism54 USB wireless driver
rt2x00lib.ko
rt2x00 library
rt61pci.ko
Ralink RT61 PCI & PCMCIA Wireless LAN driver.
rt73usb.ko
Ralink RT73 USB Wireless LAN driver.
spectrum_cs.ko
Driver for Symbol Spectrum24 Trilogy cards with firmware downloader
zd1201.ko
Driver for ZyDAS ZD1201 based USB Wireless adapters
zd1211rw.ko
USB driver for devices with the ZD1211 chip.
pcmcia.ko
PCMCIA Driver Services
aic94xx.ko
Adaptec aic94xx SAS/SATA driver
ipr.ko
IBM Power RAID SCSI Adapter Driver
libsas.ko
SAS Transport Layer
qla2xxx.ko
QLogic Fibre Channel HBA Driver
cxacru.ko
Conexant ?AccessRunner ADSL USB modem driver
speedtch.ko
Alcatel ?SpeedTouch USB driver version 1.10
ueagle-atm.ko
ADI 930/Eagle USB ADSL Modem driver
keyspan.ko
Keyspan USB to Serial Converter Driver
snd-vx-lib.ko
Common routines for Digigram VX drivers
snd-wavefront.ko
Turtle Beach Wavefront
pss.ko
Module for PSS sound cards (based on AD1848, ADSP-2115 and ESC614). This module includes control of output amplifier and synth volume of the Beethoven ADSP-16 card (this may work with other PSS cards).
snd-darla20.ko
Echoaudio Darla20 soundcards driver
snd-darla24.ko
Echoaudio Darla24 soundcards driver
snd-echo3g.ko
Echoaudio Echo3G soundcards driver
snd-gina20.ko
Echoaudio Gina20 soundcards driver
snd-gina24.ko
Echoaudio Gina24 soundcards driver
snd-indigodj.ko
Echoaudio Indigo DJ soundcards driver
snd-indigoio.ko
Echoaudio Indigo IO soundcards driver
snd-indigo.ko
Echoaudio Indigo soundcards driver
snd-layla20.ko
Echoaudio Layla20 soundcards driver
snd-layla24.ko
Echoaudio Layla24 soundcards driver
snd-mia.ko
Echoaudio Mia soundcards driver
snd-mona.ko
Echoaudio Mona soundcards driver
snd-emu10k1.ko
EMU10K1
snd-korg1212.ko
korg1212
snd-mixart.ko
Digigram miXart
snd-pcxhr.ko
Digigram pcxhr 0.8.4
snd-riptide.ko
riptide
snd-hdsp.ko
RME Hammerfall DSP
snd-maestro3.ko
ESS Maestro3 PCI
snd-ymfpci.ko
Yamaha DS-1 PCI
This page is mostly meant to expose module/firmware. It should be improved to list the matching package names.