Wikipedia
OllyDbg (named after its author, Oleh Yuschuk) is an x86 debugger that emphasizes binary code analysis, which is useful when source code is not available. It traces registers, recognizes procedures, API calls, switches, tables, constants and strings, as well as locates routines from object files and libraries. It has a friendly interface, and its functionality can be extended by third-party plugins. Version 1.10 is the final 1.x release. Version 2.0 was released in June 2010, and OllyDbg has been rewritten from the ground up in this release. The software is free of cost, but the shareware license requires users to register with the author. Although the current version of OllyDbg cannot disassemble binaries compiled for 64-bit processors, a 64-bit version of the debugger has been promised.