The Ordnance Survey JavaScript SDK offers detailed online and offline mapping capabilities of Great Britain. Maps can be customized by tweaking resolution ranges and themes.
It seems that the Where 2.0 Conference is full of pleasant surprises. Today, Yahoo announced its Placemaker platform, and Google also announced the release of their Maps Data API (our Maps Data API Profile) as the newest Google Data API.
You may already have access to your user's location and not know it yet. If you use Google's Ajax API loader to access Maps, Search or Feeds, you're one line of JavaScript away from knowing the user's country, city, or even a nearby latitude/longitude point.