Radare2

[jvoisin]

Radare2 (also known as r2) is a complete framework for reverse-engineering and analysing binaries; composed of a set of small utilities that can be used together or independently from the command line.

Built around a disassembler, it supports a large number of executable formats for different processors and operating systems. Since it doesn't have a GUI, it has a steep learning curve. Originally built around an hexadecimal editor, is has now a multitude of tools and features like ASCII-art graphs, binary diffing, patching, analysis, has native debuggers, can compile shellcodes, ..., but also bindings for several languages, like Python, javascript, Ruby, C, ...
Licensed under LGPL and written in C, it runs on every plateforms.

There is also a (completely unfair) comparison between Hopper, IDA and radare2.

http://rada.re/y/?p=download