Allows only valid characters (i.e. numbers) to be typed into a text box. Can take negative numbers and a decimal point. You can supply a callback that runs when focus is lost and the value in the text box is not a valid number.
Source code available from GitHub or a Zipped download.
Disabling pasting does not work under all browsers (fine in IE), so this feature has been removed. The old version is still available though (if anyone can figure out how to prevent paste in all browsers).