Skyhook's Loki location-finding service for browsers has a pile of sites enabled to use the technology: Skyhook Wireless has been developing its Loki plug-in and related service for some time, but this is the first big rollout of major partners that have adopted the location-finding approach into their sites. Flickr (click Find My Location), MapQuest, and WeatherBug (click Locate Me) are the marquee partners, but there's a long list of other interesting sites, too.
Skyhook's approach in the browser relies on Wi-Fi, while in smartphones Skyhook (depending on OS and implementation) can tap into Wi-Fi, cellular signals, and a GPS receiver. Skyhook uses the Wi-Fi snapshots sent by client software along with brute force driving of trucks with big antennas and highly accurate GPS to update its databases constantly.
Coordinates can be used for mapping, path finding, tracking (see where you went), pushing location to others (friends, family, colleagues), geotagging photographs, and finding nearby businesses. It's one reason why Sprint pushed so heavily to put GPS in all its data products in order to get the stickiness that comes from people getting used to being able to easily get their location.
Google, Mozilla, and the W3C are all working on this issue. Google's Gears-based Geolocation API, which works in any browser that supports Gears, was turned into an "informal" W3C proposal that's in active revision; Mozilla will support the API in Firefox 3.5 and later. Google has a non-disclosed way of collecting Wi-Fi access point data, and this data is being used by the Gears Geolocation plug-in, as well as Firefox in 3.5. Firefox will support multiple providers of location data.