Text editing: switch to Override mode
As far as I know, normal OS X text views don't support anything like that.

In TextMate 1 you can use ⌥⌘O (Edit > Mode > Overwrite Mode).

In Word 2011 for Mac you can go to View > Toolbars > Customize Toolbars &amp; Menus and drag Commands > All Commands > Overtype to a menu.