Open Source Apps
Yep! Open source apps for iPhone exist.
Publishing / Blogging Open Source Apps
WordPress is the most blogging platform. This app lets you create and edit content on your blog easily. It’s easy to manage your WordPress blog from your iPad, iPhone, or iPod touch. With WordPress for iPhone OS, you can moderate comments and create or edit posts and pages. All you need is a WordPress.com blog or a self-hosted blog running WordPress.