Posterous is a new blogging tool that forgos some of the traditional dashboards and will simply let you write the post in your favorite e-mail tool. Any photos, music, or videos you attach to your message will show up in a new blog post, which can later be administered on the Web. There's no sign-up, and in fact the first time you send a message to firstname.lastname@example.org it'll automatically create your blog and Posterous account for you.
Once registered you'll still get a way to create posts in your browser. There's a simple WYSIWYG editor, and a way to go back and edit previous creations. Like Tumblr, there's also a network of other users you can subscribe to, so their posts show up in a centralized stream. In many ways, Posterous resembles Tumblr, albeit with less structure when it comes to figuring out what you want to write about.
Writing blog posts via e-mail is definitely not a new thing. Google's Blogger product has had this for quite some time now, and will support both links and images that get pushed straight to a live post. Hosted WordPress blogs and Tumblr itself also support this feature. However, none of these products support comment forwarding, which lets you keep track of community discussion while you're away from your computer. In Posterous' case you'll get a new e-mail for every new comment with the option to reply. If you do (through your e-mail client of course) that reply will be pushed live as a threaded comment. Not too shabby.