Corky's Ordnance Survey and Google Maps

Google Maps
OS Map

The above links provide maps from each provider. Click on the map to put on a point. Once the point is on there, click on the point to get more information about that point. There is then a link to transfer to the same map from the other provider.

The OS Map also gives information about the grid reference.

The newest site, though, is Google and OS on maps.quad.co.uk