ImHex
A Hex Editor for Reverse Engineers, Programmers
...At the same time ImHex is completely free and open source under the GPLv2 license. Byte patching, patch management, Copy-Bytes-As-Array for many common programming languages, string and hex search, Bookmarks, Goto, Huge file support, variable column count, custom encodings and much more. ImHex includes a full custom C++-like pattern language that allows easy highlighting, decoding and analyzing of file formats.