ResEdit is one of the essential tools of any would-be Mac hacker - no, not the 1337 kind, the customization kind. While it doesn't quite turn applications into putty in your hands, it can come pretty darn close.
As Starrynight pointed out, it can cause a great deal of damage if you don't know what you're doing (or even if you do know what you're doing, but are unlucky enough to have the system crash in the middle of your edits). So it pays to be careful. But a master of ResEdit can do all kinds of truly awesome things. Among them:
- Modify the text of dialogs, controls, menus, and such. In some cases you can even delete or add commands, or completely reorganize them (this depends on how the app was written, though).
- Modify the layout of any window or dialog box.
- "Cracking" programs, as with a hex-editor on DOS. The major difference is that you can create backups of the code and store them within the application, for easy undoing later.
- Ripping viruses out of your system without an anti-virus program. Yes, I actually did this once.
- Refedine the graphics used in programs, such as icons, cursors, or whole pictures. This allows for some rather... interesting... Photoshop work to take place.
For more information on what ResEdit can do, check out http://www.resexcellence.com and pay particular attention to things like the splash screen section.