Disassembly is a big part of security research. It will help you dissect that Microsoft patch to discover the silently fixed bugs they don't tell you about, or more closely examine a server binary to determine why your exploit isn't working. Many debuggers are available, but IDA Pro has become the de-facto standard for the analysis of hostile code and vulnerability research. This interactive, programmable, extensible, multi-processor disassembler has a graphical interface on Windows and console interfaces on Linux and Mac OS X.
For downloads and more information,
visit the IDA Pro homepage.