Posterous: it's like Tumblr via email
It's not as if Tumblr is extraordinarily complicated, but Posterous presents some interesting possibilities. You can post your cameraphone pics and other moblogging material via e-mail, and have comments sent to you and reply to them on the go (again, via email). Also, we're not complaining about a blogging service that lets you skip past all those annoying signup processes. You already have an email account, so what do you need yet another login for?
Posterous obviously has to be careful about security, since forging email addresses isn't all that difficult. Michael Arrington offered a free TechCrunch t-shirt to the first person to forge a post on his Posterous blog, and the challenge was over pretty quickly. Posterous addresses these security concerns in their FAQ: "If we think it might not be you, we ask you to confirm the email before we post it. No matter what, you always get an email notification of every post we put online for your blog, with an easy link to remove the post if you didn't do it."