By this point, it has almost become standard for any decent-sized web service to offer an application programming interface — a way for third party developers to build applications that make use of information and distribution channels on a popular site. The most striking example is social network Facebook’s developer platform, which has spawned hundreds of thousands of applications that are now used by millions.

Not to be left out, photo-sharing site Photobucket is publicly launching its own API today. The promise is that developers will be able to access the billions of photos, slide shows and videos on its site, and integrate them into their own applications. The company has around 40 million total unique users, with users from social network MySpace comprising roughly have the traffic to the site. It is one of the larger photo-sharing sites on the web, rivaling popular photo-sharing site Flickr (which has had its own API for years) as well as Facebook, which boasts by far the largest collection of photos on the web.

The API was actually developed a few years back, but had only been privately available to commercial partners. It will let any developer do things like log in to user accounts, upload photos and videos, change metadata like titles and tags of photos, and search publicly available Photobucket content.

The company hopes to make the photos it hosts more commonplace not just on other web sites, but on desktop applications, mobile phones, home entertainment systems and more. Third parties that have already developed applications for the API include Adobe, Tivo, AOL, FotoFlexer, Intercasting, RockYou, Slide, Animoto and Snapvine. FotoFlexer, for example, is a web application (and Facebook app) that lets you make high quality, near-Photoshop quality alterations to photos. The site evolved as a place for MySpace users to store and share photos. (And in fact, it was around a year ago that News Corp. first cut off Photobucket’s access to MySpace, then bought Photobucket). Today, people also use Photobucket to host photos that are displayed on blogs, other social networks and marketplace sites like eBay and Craigslist. The service has continued to expand, adding some 15 million registered users after it was purchased. The site itself, Photobucket.com has also seen increasing traffic, with searches for photos on Photobucket.com counting as the second largest source of overall traffic after MySpace, chief executive Alex Welch tells me.

You can check out a gallery of applications that have already been built by Photobucket partners here.