This website allows you to search a wide body of digital resources relating to early modern and eighteenth-century London, and to map the results on to a fully GIS compliant version of John Rocque's 1746 map.Rocque's wonderful 1746 set of maps is a pretty accurate survey of the metropolis at that time, but lacking the kind of technology we have today it's not going to be pinpoint accurate. The map was overlaid on a modern Ordnance survey map by 'pinning' them together at 48 fixed points on the Rocque map that are still in existence today, and adjusting the old map to fit the new one. If my low-tech explanation fails to satisfy, you can find out exactly how this was done on the Mapping Methodology page.
You can use information from a number of datasets to plot all kinds of things on either of the historic maps, a modern map, Google Maps satellite view or even a blank background. I can see all kinds of applications for this; one of my particular pet projects is to map the locations of the various taverns and coffee houses where the notorious 'Fleet' marriages took place, and this site should be a big help with that.
There are plans to add a citable search URL, map export function and citation generator. They also hope to add more maps and datasets in the future, which will make it even better. In the meantime there is plenty to explore, and there is a video walkthrough to help you find your way around. You can also send suggestions and feedback through the Contact Us page. I'm sure that people will come up with plenty of applications relevant to their own research.