There are quite a few free mapping services for India on the Web but only some among them offer APIs that web publishers can extend & integrate in their own websites.
There is news now that BIG Maps has also released its application programming interface (API). The documentation is pretty comprehensive with demos but it looks like it's not all official yet as the home page of the website currently sports a "Coming soon" label.
The FAQ informs that coordinate system that BIGMaps follows is proprietary but there are ways to import and export data from/to other formats.
This is an interesting development & I hope the competition among the players results in better products.
Also see:
2 months of BigFlix