Things I've used HexEdit for:

	Debugging:
		Examine other programs' binary output for errors (AIFF, MIDI, etc.)

	Spelunking:
		Examine/Edit the contents of a MIDI file.
		Examine binary files received from the Internet to see what format 
		they are in (MacBinary, etc.)

	Reverse Engineering:
		Figure out how Kanji is encoded in a text file.
		Compare MS-Word format to RTF format.
		Examine the data fork of MS-Word Application (Owner Name is stored there
			in easily decrypted format)

	
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