Twitter

As Twitter's director of platform, Ryan Sarver, repeatedy pointed out during his presentation at this year's LeWeb that Twitter and its ecosystem of third-party developers have a highly symbiotic relationship. Twitter's APIs are still rate-limited and Twitter won't make the full firehose of feeds available to all of its developers until early 2010. There can be no doubt, however, that Twitter has managed to create one of the most vibrant developer communities around its platform.

Over the course of the year, Twitter introduced a number of new APIs, including a geolocation API that makes it easy to attach geodata to a tweet, and the controversial retweet API. With Twitter Connect, Twitter also released a competitor to Facebook Connect, although this tool hasn't found widespread adoption yet.

Facebook

Just like Twitter, Facebook's success has increasingly become dependent on the third-party ecosystem that has grown up around the Facebook platform. According to Ethan Beard, who manages development of the Facebook Platform, over 500,00 applications have been developed on top of the Facebook platform and over 250 million users use at least one of these.

The Facebook platform is not just about letting users play games like Farmville and or novelty apps like SuperPoke. Facebook Connect, for example, is becoming increasingly popular as a sign-in mechanism on third-party sites. Over 80,000 sites now utilize Facebook Connect, 60 million Facebook users use it and two-thirds of all the sites in the Comscore 100 index currently use it.

WordPress.org

Without any doubt, the world of blogging would look very different today if it wasn't for Automattic's open-source version of WordPress. The core open-source WordPress project is driven by a passionate group of developers, but a majority of development happens in the community that builds plugins for WordPress.

Earlier this year, WordPress released version 2.8 of its self-hosted product - a major overhaul of its core product. It includes a new interface and new options for plugin developers, including the ability to install and search for plugins from within the admin interface. Among some of the most popular WordPress plugins are an image gallery, an SEO tool and an analytics package.

iPhone

We admit that our choice of the iPhone as a top web platform could be somewhat controversial. After all, Apple's less than efficient approval system and the closed nature of the platform have even led some developers to abandon development for the iPhone altogether.

There can be little doubt, however, that Apple has created one of the most successful mobile developer programs. The App Store now features over 100,000 applications and an increasing number of web services now offer versions of their products that are specifically geared towards the iPhone's Safari browser.

This year, Apple extended the SDK with a number of new and improved features when it released version 3.0 of the iPhone OS. These updates include better support for 3D gaming, augmented reality apps, easier access to maps, in-app purchases and support for push notifications.

Android

Android came of age in 2009. It was still a relatively unknown mobile operating system in the consumers' conscience in 2008, but this year not only saw a large increase in developer activity, but also a strong interest in Android phones like the Motorola Droid.

Thanks to the open-source nature of the project, Android made it easy for developers of augmented reality applications to test their ideas long before Apple offered the necessary APIs on its platform. While the Motorola Droid features Android 2.0 already, most other manufacturers don't offer this update for their customers yet.

Next page: Top 10 Web Platforms of 2009, 6-10