HxD Hex Editor
Full user review
"One of my favorite tools"
Complete access to the hard drive and ram... if you what you're doing.
Doesn't hold your hand, just gives you what you want and fast.
Only runs on windows?
This program has saved my computer many times. Most notably allowing me to reconstruct the boot sector of my hard drive. It is invaluable when trying to recover data off of failed disks and is generally one of the applications I install first when I reinstall Windows.
(If you couldn't tell, this program is amazing)