I think I have found the easiest way to blog. It is Posterous, a service much like Tumblr, and other micro-blogging services and other regular services. In fact you can easily convert a WordPress site to something that looks like Tumblr or Posterous.
But with that said, most services still require that you register and sign in before you can start using it. Instead, Posterous requires only one thing, an e-mail.
All you have to do to get started using Posterous is to send an e-mail to email@example.com.
It is so simple, I setup my Mom to use it so she can share all her jokes. Lucky you! Not only do I get her spam, the rest of the world does too, and you can find it at http://spamfrommom.posterous.com/ Now I have to show her some basic editing techniques so the paragraphs look better.
But if you're looking for a simple way to blog, this might just be the thing for you. One e-mail gets you started and you can continue to send posts, photos, video, or whatever from that e-mail address.
If you want to get a little more advanced, then you can register and put a password to your account, which can also be shared with multiple people. A beautiful and simple service.