Free Hex Editor "frhed"
frhed is a basic free hex editor available at: http://www.kibria.de/frhed.html
This is _not_ the malware with the same name.
Instead, it's free software released under the GPL and includes source. From the homepage:
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* Open Source! Distributed under GNU General Public License
* Native 32 bit Windows application: long filenames supported, editable file size limited only by available system memory.
* Small program size (ca. 100K), loads fast.
* Cut, copy & paste binary values.