Well I discovered this a couple days ago and am having fun. I've done maps of the neighborhood over the last 21 years, for example this one with acrylic paint on canvas: http://www.flickr.com/photos/edgehill/5193556464/
I have worked with custom google maps, and that's handy but not very good looking and all you can do is put those little balloon tacks on, no way to label anything. You can draw but no accessible attributes... OSM is better looking but very limited in what you can get to render, and there seems to be no way to add custom annotations, only standard geographic features.
I'm interested in mapping the parks, stairways, paths and natural areas. It would be great to map more of the land use also like the retail districts are highlighted, similar to the one I linked above. SFprospector has that zoning info, which I've done a map kind of like that before in AutoCAD years ago. Theoretically the city data might be made available for direct import. I would correct it to reflect actual conditions though, so the result would be something that gives you a picture of the texture of the landscape, whether an area is single family homes or big apartment blocks, industrial, etc. Same for open spaces; forest, brush, grassland, irrigated park, etc.