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While I do most of my blogging on my computer, uploading the posts and images directly through the control panel. However, from time to time I will write a post while on the go, using my iPad or even (shudder) my iPhone to write and post the update.
Whenever I do that, my choice of app is easy; because all my blogging is done on blogs using WordPress, I use the official WordPress app, which lets me do many things, such as comment management, tag and category management, and, of course, posting, scheduling and editing blog posts:
The WordPress app uses the XML-RPC framework to talk to the blog itself, and allows you to do pretty much anything that you want to do with regards to comments, posts and pages, as well as reading usage stats for the site. It allows you to configure several sites, and you can readily choose between them from the app: