Here's an essential Google Maps mashup for sailors and anglers that displays tidal information as well as sunrise and sunset times, moon phases and the best fishing times.
The blue map markers show the locations where tide data is available. The closer in you zoom the map, the more markers are shown. Each blue marker has an arrow showing whether the tide is currently coming in or out. The dark blue approximates the level of the water at the current time. These markers continue to update while the map is displayed.
Hovering your mouse over the blue marker reveals details of the place, the current time there, and when the next high or low tide will occur. Clicking on the marker displays a tide table and a graph.