I would also have pointed to Multipmap, but here are two more that might be worth bookmarking for yourself or whoever reads the thread.
The first definitely comes under "something to bookmark for next year"
OpenStreetMap
This is an editable or wiki style map, set up a couple of years ago as a direct response to Ordinance Surveys monopoly of mapping data in the UK. It started up, as the name suggests, to map out streets and was dominated by cyclists. But there are no restrictions on what can be mapped and open areas are beginning to be mapped.
The data provided is freely available to anyone (the whole point) and several maps are being created using the data. The website site shows one version of the map using a map renderer called mapnik.
The great thing about this map is it allows you to map out your local patch and make the information available to other people, something which would be very useful on this website. There are still huge holes in this map, which is what everyone spots first, but you can fill them in. There is a problem at the moment with a need to sort out ways of mapping habitat, and then getting the main map renderer to show them.
The following video gives and idea about how the map is "filling in"
OSM 2008: A Year of Edits on Vimeo
The best part for me are the free maps some people are making for Garmin GPSr's. I can go out for a walk with a free detailed map on my GPS which includes contour lines, and I can update this map every few weeks.
FREE UK Maps Home ?(talkytoasteruk)?
But as I said at the beginning OpenStreetMap is something for later, and I expect it has a good chance of becoming the main map for many uses
The second site is useful for getting Grid Coordinates. It shows the OS map and other maps, side by side, and was created to allow people to create gpx files for their GPS recievers.
Where is the path (or street)
There is one big problem, this free site has a limit on how many OS tiles it can take from OS each day, and it usually has to stop showing OS tiles by the late afternoon. But when its working its useful to compare OS maps along side GoogleMaps or OpenStreetMap. It also can show a 1940's OS map.