Mapnificent is a Google Maps mashup that shows how far you can travel on public transport for any given time period in a number of worldwide cities. The Berlin map has been around for a while but Mapnificent now works in London, a large number of US cities, in Auckland and in Perth.
To navigate to a city just click on its map marker. Next click on any location in your chosen city, set the length of time and the map will add a layer to the map showing the areas that you can travel to in that time from your location.
This map is an amazing resource for anyone who is looking to buy a property and wants to work out the areas within a set commuting time from their work place. As an added bonus you can use OSM map tiles instead of Google Maps or you can view a 'night' view created with the Google Maps API V3 Styled Maps function.