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OllyDBG
From NetSec
OllyDBG is an x86 program used for userland (ring 3) debugging of applications. Though in recent years it's purpose has slowly changed to a primarily reverse engineering nature. OllyDBG is used very often in reversing maleware, applications, and even windows DLLs.
From the OllyDBG website:
OllyDbg is a 32-bit assembler level analysing debugger for Microsoft® Windows®. Emphasis on binary code analysis makes it particularly useful in cases where source is unavailable. OllyDbg is a shareware, but you can download and use it for free.