Transit Time Map
WalkScore have produced a Google Map mashup that can show you how far you can travel in San Francisco, Portland or Seattle on public transport in 15, 30 and 45 minutes. The map can show you how far you can travel on public transit from a given location and at any time of day.
The map was created using the Graphserver trip planner, which in turn uses data from OpenStreetMap.
Where Can I Live? does something similar in London. Where Can I Live? mashes up London tube journey times with properties for sale. The map therefore allows you to find properties within a set commute time from your work.