Ensure that the character entered is numeric. Useful for checkout quantity fields.

jquery.force-keypress-numeric.js

$('input').keydown(function(e){
    // Make some allowances for: delete, backspace, tab and escape
    if ( e.keyCode == 46 || e.keyCode == 8 || e.keyCode == 9 || e.keyCode == 27 || 
       // Allow: Ctrl+A
       ( e.keyCode == 65 && e.ctrlKey === true ) || 
       // Allow: home, end, left, right
       ( e.keyCode >= 35 && e.keyCode <= 39 )) {
       // Let it happen, don't do anything
       return;
    }
    else {
        // Ensure that it's a number else stop the keypress
        if (e.shiftKey || (e.keyCode < 48 || e.keyCode > 57) && (e.keyCode < 96 || e.keyCode > 105 )) {
            e.preventDefault();
        }
    }
});