There's no doubt that the plethora of third-party developer platforms and APIs that exist today have given developers a considerable number of opportunities to build applications that can quickly made available to millions upon millions of people.
Here is a list of some of the more popular platforms and APIs that developers have successfully leveraged to build popular applications - some of which even reportedly generate significant revenues.
Promoted as "the most scalable, secure and popular on-demand platform in the world today," Force.com is Salesforce's 'Platform as a Service' offering that is designed to enable developers to create and deliver on-demand business applications.
Google Maps API
The Google Maps API is probably one of the most popular APIs on the internet today and for good reason - there are a lot of ways to "mash up" Google Maps with other services.
Amazon Web Services
Amazon Web Services give developers "direct access to Amazon's robust technology platform." This includes infrastructure services, payment and billing services, on-demand workforce, web search and information services and Amazon's Associate services.
No other mobile phone has caused as big a splash as the iPhone and for developers looking to write applications that iPhone users can install on their iPhones, the iPhone SDK coupled with Apple's iPhone Developer program enables them to do just that.
The world's largest social network offers a platform that enables developers to build applications that its 100+ million members can use.
Mozilla Plugins and Extensions
Interested in adding new functionality to Firefox and Thunderbird? Mozilla Plugins and Extensions enable just that.
Google's answer to Facebook's platform gives developers the opportunity to write applications that are accessible via third-party OpenSocial partners, including popular services such as MySpace, LinkedIn and Plaxo.
MySpace Developer Platform
Like the Facebook Platform, the MySpace Platform gives developers the opportunity to build applications that MySpace's 100+ million members can use.
The Twitter API allows developers to write applications that tap into Twitter's growing microblog community.
Available for non-commercial use (and commercial use by arrangement), Flickr Services give developers the ability to interact with the popular Flickr photo sharing service.
The YouTube offer developers the opportunity to work with the world's most popular online video website.
Designed to be "the first complete, open and free mobile platform," Google Android has received a lot of buzz and given the involvement of major technology and mobile companies, could be one of the "next big things" for developers in search of a platform to build on.
These are but a handful of popular platforms and APIs available to developers today. There are literally thousands more out there that provide interesting opportunities to developers.
ProgrammableWeb has a great directory worth checking out for more ideas.