IDA Pro : A Windows or Linux disassembler and debugger
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 disassemblers 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 now supports Linux (console mode) as well as Windows.