Posterous releases directory of themes
[Update - As noted in the comments, it appears that themes.posterous.com is not an official directory of themes as the title of this post suggests, but rather is a 3rd-party theme directory that cleverly grabbed the "themes" subdomain at posterous.com.]
The super-simple blogging platform Posterous has released a theme directory, offering up eight new themes in addition to the five default themes offered right from within Posterous's management interface. To download the new themes you must be a member of posterous.us, a forum dedicated to helping users make the most of their Posterous blog.
Once you create your posterous.us account, you can download the theme, which amounts to simply a text file containing the code for a CSS stylesheet. Copy the text and paste it into the management interface for your Posterous blog in the Theme > Advanced section, apply the change, and you're all set.