Recuva
Very effective in restoring deleted files. Extremely popular.
FreeUndelete
No frills, just focused on the zen of file recovery.
ADRC Data Recovery Software
The Renaissance Man of free data recovery. Not as popular as Recuva but still has its own fans.
Pandora Recovery
An easy to use yet powerful file recovery application.
PhotoRec
Designed specifically for the recovery of lost photographs, PhotoRec is very powerful free software.
TestDisk
Incredibly powerful free software for recovering whole lost partitions and making non-bootable disks bootable again.