hacker.org

Here some tools which proved useful to me to solve challenges.

For general purposes:
A Freepascal compiler with powerful IDE at http://lazarus.freepascal.org/ which is even more powerful and easy to use than Delphi.

To analyze images:
Stegsolver by Caesum available at http://www.wechall.net/download/12/Stegsolve_jar

To decompile Java:
http://java.decompiler.free.fr/

To factor very big numbers:
MSieve at http://sourceforge.net/projects/msieve/

To simulate Game of Life:
http://sourceforge.net/projects/golly/

My warrior for the Dungeon series was in Bash. I use Cygwin as Unix shell, I have also a combination with Virtualbox and Fedora image.

To prototype calculations with big numbers: Python, Maple (not free!) and Freepascal with MPArith library if speed is required.

With GCC i compiled some C and C++ stuff, but only for particular cases...

GIMP helped me sometimes as well...

Finally a diary with pencil and paper to design attacks to the single challenges!

And what are your favorite tools?

I submit Mathematica (again, not free, but useful for almost every challenge!) and WolframAlpha, as well as Colonel Dump's HVM IDE (though I did eventually write an HVM compiler in Mathematica, following their SymbolicC style of design).

Some of the software which I use:

MinGW/MSYS: http://www.mingw.org/
to compile C code under Windows

IrfanView: http://www.irfanview.net/
can not only show pictures, but also a hexadecimal representation of them, which can be useful

TweakPNG: http://entropymine.com/jason/tweakpng/
useful for challenges involving PNG files

Notepad++: http://notepad-plus-plus.org/
possibly the best free text editor for Windows

IDA (Interactive DisAssembler): https://www.hex-rays.com/products/ida/s ... load.shtml
to disassemble machine code, the free version has limitations (e.g. 64bit not supported)