Chameleon
Drag-and-drop tool for changing filetype and creator codes

MuchoFileInfo
Rename, change filetype and creator codes, change finder flags.

ResEdit
One of the most useful ‘hacker’ level tools around. Dangerous when misused.

ResKnife
Resource editor with extensive Mac OS X support

ShrinkWrap
Create and mount disk image files with ease

TomeViewer
Dig into installer 'tome' files and extract individual items