Here are some free (mostly open source) hex editors:
- HexEdit from Expert Commercial Software: Some free and with-source versions, also newer commercial versions. Looks full-featured.
- Hexplorer: Sourceforge project under GPL license, has some interesting features like pixel view and colour schemes.
- frhed from Raihan Kibria: Now a SourceForge project under GPL taking on some features and fixes from other derivatives like HexSource below.
- HexSource from brixoft: Updates to frhed under the GPL license.
- HexEdit from James Brown (a.k.a. Catch22 Productions): Nice hex editor with a file size limit of 4Gb (on 32-bit windows). Not open source, but there is a tutorial showing development of a large file hex viewer component which is better than nothing!
At university we use WinHex, which isn’t free (especially the Forensic version that we use) but it’s good.