What OS X tweaks, hacks, or modifications couldn't you live without?
~/Library/KeyBindings/DefaultKeyBinding.dict

A property list that enables you to bind keystrokes to sequences of Objective-C methods. It can be used to define text editing macros that work in most standard text views.

{
    // copy paragraph (option-c)
    "~c" = (selectParagraph:, copy:);

    // convert selection to lowercase (option-minus)
    "~-" = (lowercaseWord:, moveForward:);

    // insert line below (command-return)
    "@\r" = (moveToEndOfParagraph:, insertNewline:,
    deleteToBeginningOfParagraph:);

    // insert an XML tag (option-shift-x)
    "~X" = (moveWordBackwardAndModifySelection:, setMark:, deleteToMark:,
    insertText:, "&lt;&gt;&lt;/&gt;", moveBackward:, yank:, moveWordBackward:,
    moveBackward:, moveBackward:, moveBackward:, yank:, moveForward:);
}


To get started, make ~/Library/KeyBindings/ and save a property list like the one above as DefaultKeyBinding.dict.

More information:


Cocoa Text System
lri.me/keybindings
github.com/ttscoff/KeyBindings