"For Windows, Keytweak and Microsoft Keyboard Layout Creator do what I need (Keytweak is fast and easy with a nice UI, MSKLC is more involved and requires compiling keyboard layouts, and is occasionally flaky)...
For OS X, I'd highly recommend Ukelele:
Ukelele is a Unicode Keyboard Layout Editor for Mac OS X versions 10.2 and later. Ukelele aims to simplify keyboard layout editing by providing a graphical interface to .keylayout files, where the desired characters can simply be dragged onto keys as needed. (The Character Palette, available in the Input menu if it has been enabled in System Preferences, is a great place to find the characters.) In addition to simple assignment of single character codes to keys, Ukelele can assign multiple-character strings and can create ""dead keys"", where a keystroke sets a new state that modifies the output of the following keystroke.
For full-power keyboard assignment in OS X, Quicksilver triggers and QS in general."
