Hex Editors

Hex editors allow you to inspect and edit binary files in hex mode

XVI32 is an excellent freeware hex editor for Windows. It can manipulate extremely large files, and could prove very helpful if you have to edit large text or binary files.
[OS: Windows] [Price: Free]
010 Editor
010 Editor is a professional-grade hex editor designed to quickly and easily edit any binary file or drive of unlimited size. It features Binary Templates which allow you to understand any binary file format. Allows you to analyze and edit binary data with powerful tools including Find, Replace, Find in Files, Binary Comparisons, Checksum/Hash Algorithms, Histograms, etc.
[OS: Windows] [Price: $49.95]

FlexHEX is an advanced hex editor designed to edit binary files, OLE compound files, logical disks, and physical drives. With FlexHEX you can modify, inspect, insert, search, or replace binary, ASCII, or UNICODE data. While providing a full set of features for editing raw hex bytes, FlexHEX also supports editing binary data as typed values in a number of formats. The data conversion functions let you easily import or export binary data, or just drag-and-drop them to/from another application. In addition to the powerful editing functions, FlexHEX includes an unlimited Undo/Redo list, so you are never at risk of losing your data because of a typing error. A rich set of navigation and tracking functions make browsing binary data quick and easy. Bookmarks, the modified area list, the jump history, and other convenient tools make inspecting binary data easy and straightforward.
[OS: Windows] [Price: $39.95]