Disk imager (for Linux):

recoverdm : again, a 'dd' like tool. Seems that it had a special mode for CDROM.
myrescue : a sort of 'dd' like tool, but specialized in harddisk data copy (better handle of damaged areas).
dcfldd : dcfldd is an enhanced version of GNU dd with features useful for forensics and security, and AIR the GUI front-end to dd/dcfldd.
safecopy : safecopy is a data recovery tool which tries to extract as much data as possible from a seekable, but problematic (i.e. damaged sectors) source - like floppy drives, harddisk partitions, CDs, ..., where other tools like dd would fail doe to I/O errors.
badblock-guess : badblock-guess will recover all readable sectors of the disk in minimal time while trying to prevent disk read retrying head recalibrations.
dd_rescue and ddrescue
dd_rhelp : dd_rhelp is a bash script that uses dd_rescue.

Disk imager (for Windows):

WinImage
Rawrite and related (floppy) disk imaging programs.

Partition tools :

gpart : primary partition table "guesser" tool.
TestDisk : check and undelete partition (a lot of partition types are recognized).

Filesystem tools :

e2salvage
e2extract
e2undel
fatback : FAT recovering tool (available from the Biatchux bootable CD project for forensic/analysis/data recovery/virus scanning/...
FScat : FSCat is a utility for dumping filesystems offline.
Ext2 Installable File System For Windows provides Windows NT4.0/2000/XP with full access to Linux Ext2 volumes (read access and write access).
ReiserDriver is an installable File System Driver that allows ReiserFS partitions to be accessed under Windows
FFS File System Driver for Windows 2000/XP/2003, allows you to get a read access to FreeBSD, NetBSD and OpenBSD FFS(UFS) partitions.
ElcomSoft's Advanced EFS Data Recovery and its online documentation.

File tools (identification, recovery, degraded mode read, consistency check, ...) :

file : the famous file Unix command
foremost : recover files based on their headers and footers (work on raw data files from dd for example).
gzip recovery tookit : self explanatory...
photorec : PhotoRec is a little tool to recover pictures from digital camera memory (you could also check my gepj-fat utility to recover JPEG files from FAT12/FAT16 filesystems).

Various tools :

Hex editors and viewers : hexdump from util-linux, KDE's khexedit, hexedit
Hachoir is a library written in Python which allows to see and edit a binary files (or any binary stream) field per field. It can be used to extract some informations (eg. metadatas), edit some fields...

Forensic suites :

The Sleuth Kit & Autopsy : Forensic tools for Linux and other Unixes (check the excellent Links part).
TCT (The Coroner's Toolkit), and don't miss from Wietse Venema and Dan Farmer Forensic Discovery" online version book.

Linux distributions / Live CD / NAS :

NASLite : single floppy disk based Network Attached Storage (NAS) Server Operating Systems designed to transform a basic computer into a dedicated file server.
SMART : SMART data forensic suite and CDROM from ASR Data.
System Rescue CD : is a linux system on a bootable cdrom for repairing your system and your data after a crash.
Helix : Knoppix based forensic Live CD.
RIP : Recovery Is Possible Linux rescue system.
PLD RescueCD
Ultimate Boot CD : a bootable CD containing many floppy based utilities for hard drives, partitions, filesystems, antivirus, mainboard...
KNOPPIX, Ubuntu Live, SLAX,...

Free Microsoft Windows software :

Handy Recovery 1.0 at the bottom of the page.
PC INSPECTOR.
Linux Recovery and Linux Reader for ext2/ext3 filesystems, from DiskInternals.

Commercial Microsoft Windows software :

R-Studio Data Recovery is a family of powerful and cost-effective undelete and data recovery software..
GetDataBack : do-it-yourself data recovery software.
Easy Recovery : data recovering tool for Microsoft Windows filesystems.
Active@ Partition Recovery : MBR and partition recovery tool.

Hardware :

PC-3000 : from ACE Data Recovery Engineering, developp software and hardware solutions for harddrive data recovery.
A BIG forum forum about Data Recovery and Hard Drive Repair.
Anatomy of a Hard Disk Drive
HDD Firmware Center.
Actionfront Recovering Unrecoverable Data whitepaper that focus on their SignalTrace technologie. Hardware oriented but very interesting.
Head Stack Replacement: Questions and Answers
Seagate RS-232 adapter schematic (don't know what it could really be used for)

Hardrive password unlocker:

HDD Unlock Wizard is a user-friendly application which allows you to easily unlock IDE and SATA Hard disk drives.
HDD password removal
AFF Repair Station is data recovery software which allows you to diagnose and repair system area problems of hard disk drives, and to remove passwords from drives locked with an ATA-password.

Wipe and Erase data :

Can Intelligence Agencies Read Overwritten Data? an answer to the famous Secure Deletion of Data from Magnetic and Solid-State Memory Gutmann's paper.
There's also Dan Kamiski Thoughs on Secure Deletion part 1 and part 2
DBAN : Darik's Boot and Nuke ("DBAN") is a self-contained boot floppy that securely wipes the hard disks of most computers.
CMRR Protocols for Disk Drive Secure Erase (and the paper).
Protecting Sensitive Data : Researchers Develop Fail-Safe Techniques for Erasing Magnetic Storage Media.

Prevention :

dvdisaster : dvdisaster provides a margin of safety against data loss on CD and DVD media caused by aging or scratches.
Clonezilla is a sort of Norton Ghost (much more like Symantec Ghost Corporate Edition), that is able to massively clone many computers simutaneously.
parchive : a great archive tool with special file format that allows easy recovery by adding self protection in case of corruption in missing parts.
And naturally... backups !

Documentation on filesystems internals and file formats :

The structure of the Reiser file system

Various :

Bad Sector Blues : The Dreaded "error=0x40 { UncorrectableError }"
FileDisk is a virtual disk driver for Windows NT/2000/XP that uses one or more files to emulate physical disks ().
Mount Image Pro Computer Forensics software to mount EnCase, DD and SMART evidence files as virtual drives on your file system.
DFTT : Digital Forensics Tool Testing Images
Recovery Tools section at Freshmeat.
Data Recovery : a lot of links related to data recovery at dmoz directory.
Wikipedia Data recovery article.
An interesting paper that "clear up misconception prominent in the storage community" about SCSI and ATA.
A compilation of filesystem documentation
Something you shouldn't have to care about.
Hard Drive Recovery blog (doesn't seem active since june 2005).
National Software Reference Library (NSRL) provide a collection of file hashes from numerous commercial softwares.