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If you're just remapping modifier keys, the Keyboard prefs pane works well (not sure if it's part of Leopard, though).

If you want to map something to ESC, try PCKeyboardHack: http://pqrs.org/macosx/keyremap4macbook/extra.html

You can remap the modifier keys (Caps Lock, Alt, Cmd) from the preferences pannel, in Keyboard. There's a button at the bottom, called "Modifier Keys..."

This is true but that utility won't let you remap Caps to Esc.

Very good Mac keyboard remapper: http://scripts.sil.org/ukelele