Radare2
 

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.

Radare2 is holding its second Radare Summer of Code this summer! Ho, and we've got also a slot for a GSoC student.

So if you want to spend your summer writing code for an open-source reverse-engineering framework while being paid, please apply ;)

Radare2 Windows installer: