Google Maps, if you haven't come across it, is absolutely incredible. Only works for the US at the moment, though.
Fortunately for me, I am going to San Francisco for the Open Source Business Conference 2005 in April. So I've been busy finding out about hotels near the venue, what's in the area, and so on. Google Maps is phenomenally useful for this.
When I first started using it, I thought "where are all the buttons?" and "how do I do anything with it?". But all the functionality is in the map itself, and it reacts like a desktop application: you can double click to centre the map on a location, drag it around with the mouse, etc., all stuff it never even occurred to me to use with a web application (I am old skool, after all: my first browser was Lynx). Then you can drill-down into a section of the map, and search for hotels, restaurants, cinemas and so on in the local vicinity; their locations then pop up as flags on the map you can click on. (Looks like lots of Italian places near where I'm staying :) Extraordinary.
If you're interested in the so-called Ajax technology this is based on, one of the best articles is at http://jgwebber.blogspot.com/2005/02/mapping-google.html. (Unfortunately, Ajax is also the name of a household cleaner, which takes the edge off the excitement for me.)