http://leafletjs.com/ 

I came across this today while attempting to do a mashup between Os maps (bing), NLS maps, and Welsh lidar maps. These three work, apart from the free NLS map being a bit offset and limited to not particularly old maps. I did have a peep at getting BGS as a layer, but I don't think they have a tile server, although a bit of screen scraping and bodging might suffice.

Anyway, the Leaflet api seems to be a lot easier to use than Openlayers, so just a heads up.

Robin

http://www.toadstone.com/pdmhs/youlgreave_map/ 

You import your (in this case a converted PDF) into the Maptiler select how you want to proceed and it creates the necessary map tiles and fading layer.

Its also handy for creating your own scale maps further more you can create definitive locations/markers on your custom map. The fire extinguishers here are held in a json file that also has positional data and other info that can be displayed in the associated popup.
http://www.how2get2.co.uk/mmah/fire/fire_ex_level0/ 
This particular map is for touch on a mobile hence the central cursor i.e. move the map to the centre cross hairs which gives the positional readout to the bottom left of the map. If using on a desktop you can use the mouse pointer position code instead. Both are leaflet plugins.

Well worth using.

I note that the Youlgreave map api was out of date so I had to get another api key off Google. There used to be free api's but it would appear now you have to register as a developer if you want to layer your own data etc.

Cheers, Peter.

Not played with overlays yet. I like that Youlegreave one.

At the moment I've got a mashup of the following each with separate transparency control:

Bing OS 1:25,000
Mabbox aerial
Mapbox road
BGS Solid geology
BGS Soil
English and Welsh lidar
NLS 1949 OS Map