Open Mapping System?

I’ve been playing with Google Maps a bit for a talk I’m giving on Monday. It’s addictive stuff.
But I’d like to be able to use alternate kinds of maps. E.g. plotting Samuel Pepys’s diaries on a historical map of London, or points on a sonar map of the sea, or views of MMPORG maps.
It’s lead to me to wonder: has anyone written a general purpose front-end similar to Google Maps? Basically all of the Ajax/DHTML magic, but without the actual Google supplied tiles? It’d be interesting to have that and then be able to plugin in different server-supplied tiles, perhaps on the fly (e.g. radar vs satellite images).
The whole client-side framework would be portable I think. It would be an interesting way to explore any data set that can be visualised in two dimensions.
Anyone already done or doing this? Drop me a mail if you are.