WordPress features a very rich XML-RPC interface that you can work with from external applications. It provides you access to most of the functionality you'd have directly in the admin - write posts, edit posts, edit comments, create/edit categories, manage site options, upload files, etc.
As a matter of fact, certain third-party applications are already using this functionality to allow you to manage WordPress remotely - a good example is Windows Live Writer. It works for both self-hosted (.org) sites and sites hosted on WordPress.com.
So if you need to get posts from WordPress, I'd recommend following API calls:
- metaWeblog.getPost - for retrieving a single post based on its ID.
- metaWeblog.getRecentPosts - for retrieving a list of recently published posts
You can also create/edit posts with these API calls:
- metaWeblog.newPost - create a new post
- metaWeblog.editPost - edit an existing post
There's more information about creating your own application (and an explanation as to why WordPress supports third-party APIs like metaWeblog) in the Codex.
I've also written tutorials on how to use the API and documentation specific to making/parsing metaWeblog API requests.