iBored is a tool to view and edit blocks (sectors) of disks and related files (e.g., image files).

I've used it myself to repair iPods that had been borked by installing iPodLinux on them, rescue the most possible data from hard disks with defective sectors, analysed Flash (.flv) files, and much more. It can even access disks remotely over internet.

If you know your bits and bytes, have a look at this tool. It runs on OS X, Windows, and Linux, thanks to it being made with REALbasic.

Features

Can view disks in custom block sizes.
When modifying data, a "journal" file is written to your desktop that contains the previous data of the altered blocks, and it can be used to undo your changes.
Can view partitions and other subranges as containers with their own start, length and block size.
Can be re-launched with admin (root) rights to access non-removable disks) (Mac OS X only).
Detects connected iPods, showing their firmware partition contents.
Can access disks remotely over a network connection.
Can save a range of blocks to a file and write a file back to disk.
Has a powerful template system to view and analyse disk structures.

Limitations

Some operations are only implemented for OS X, not Windows nor Linux.
Hardly any testing has been done on Linux and Windows, lately. If basic operations (read, modify, write) fail, let me know!
Templates are limited, and some operations may only work under certain conditions - I've often only implemented them as far as I needed them for a particular case.

Download Old Versions of iBored: 1.1.19