Interactive Disassembler
The Interactive Disassembler, more commonly known as simply IDA, is a disassembler widely used for reverse engineering. It supports a variety of executable formats for different processors and operating systems.
Although IDA performs a large degree automatic reverse-engineering to a certain extent, leveraging cross-references between code sections, knowledge of parameters of API calls, and other information, it is focused on being interactive. A typical IDA user will begin with an automatically generated diassembly listing and then rename, annotate, or otherwise add information to the listing, until it becomes clear what it does, creating an effective reverse-engineering.
Although at one time IDA was distributed freely by its creator in Russia, it was later turned into a commercial product by DataRescue, a Belgian company, who currently maintains and supports an improved version they call IDA Pro.