Mapping the past in the present day

I am constantly on the look out for new tools on the Internet to use for research and just general mucking about. One such service I have come across is Where is the path, which gives you various images from 'today' alongside old OS maps.
This is the effect you get when looking at an area based on Milehouse:

 

On the left we see the old OS map which can be zoomed in and out, although the quality suffers on some maps when you zoom in too much, and on the right we see the present day image. In this case I am using OS satellite imagery. You can also see the same areas in Google Earth (if you have the plug in installed) of plain Google Maps or satellite image, or a few other maps. The blue square on the left is the area covered by the image on the right and also the blue cross hatch matches where you place your cursor on the new image. This is useful when tracking old lanes against the modern landscape.

Thanks to the less than helpful attitude of OS there is a strict limit to the number of times the service can be called upon during the day, but other than that this is a great way of discovering the past by looking on the Internet. If this is what can be put together by one individual working within the confines of extremely limiting constraints, you can only begin to imagine what OS could do if they put their minds to it, or if they just allowed people better access to the information, which lets not forget, has been paid for by taxpayers over the years!