http://wevther.com is about the coolest idea I have seen for a while but being from the other side of the pond I couldn't understand the temps in fahrenheit.

Celsius_To_Fahrenheit_Converter.coffee

# Usage:
#  temp_converter.convertTo('f')
#  temp_converter.convertTo('c')

temp_converter = 
  temps: ['.temphigh', '.templow']
  currentUnit: 'f'

  toCelsius: (f)->
    @currentUnit = 'c'
    Math.round((parseInt(f)-32) * (5/9)) + '°'

  toFahrenheit: (c)->
    @currentUnit = 'f'
    Math.round(parseInt(c) * (9/5) + 32) + '°'

  convertTo: (unit)->
    if unit is @currentUnit then return false
    for temp in @temps
      $temp = $(temp)
      $temp.text switch unit
        when 'c' then @toCelsius $temp.text()
        when 'f' then @toFahrenheit $temp.text()

Celsius_To_Fahrenheit_Converter.js

// Usage:
//  temp_converter.convertTo('f')
//  temp_converter.convertTo('c')

var temp_converter = {
  temps: ['.temphigh', '.templow'],
  currentUnit: 'f',
  toCelsius: function(f) {
    this.currentUnit = 'c';
    return Math.round((parseInt(f) - 32) * (5 / 9)) + '°';
  },
  toFahrenheit: function(c) {
    this.currentUnit = 'f';
    return Math.round(parseInt(c) * (9 / 5) + 32) + '°';
  },
  convertTo: function(unit) {
    var $temp, temp, _i, _len, _ref;
    if (unit === this.currentUnit) {
      return false;
    }
    _ref = this.temps;
    for (_i = 0, _len = _ref.length; _i < _len; _i++) {
      temp = _ref[_i];
      $temp = $(temp);
      $temp.text((function() {
        switch (unit) {
          case 'c':
            return this.toCelsius($temp.text());
          case 'f':
            return this.toFahrenheit($temp.text());
        }
      }).call(this));
    }
  }
};