WordPress.org

Provides a new section in the superadmin area under 'Sites' called 'Dead'.

Lists blogs, just like 'All Sites', but provides filters specific to finding dead blogs. You can filter by:

1. Last updated
2. Number of posts
3. Number of pages
4. If the blog still has the default theme

Also offers a bulk email feature where you can email all admins of a set of blogs with some codes for personalization.

Note that you might need to be using a modern browser in order for all features to work.