Another nice thing about Boilerplate is the amount of third-party modification and integration. WordPress developers can choose from a whole slew of themes built on Boilerplate; various third-party forks of Boilerplate have taken the basic design and built on top of it in various directions; and countless integrations exist for other frameworks -- Rails, the Google App Engine, Drupal, Django, CakePHP, Node.js, and a whole lot more. The documentation is a plus too. The project's maintainers have gone out of their way to create a solid, step-by-step guide to most every aspect of deploying and modifying an HTML5 Boilerplate-based site, so you're not just left to unpack a .ZIP file and cobble the pieces together yourself.
I mentioned how Boilerplate comes off as being a bit more of a blank slate than other layout frameworks, but that doesn't mean it's that much less useful. It just demands more of an active hand if you're creating something with it. I liked the canned examples provided by Bootstrap in the standard download because they give you a much more immediate, hands-on idea of what can be done with the framework.
320 and Up Most design frameworks begin at the desktop level, then degrade gracefully for a smaller browser. 320 and Up works the other way around. It starts with a design meant for small screens (320 pixels, hence the name), then scales upward to a maximum of 1,382 pixels. To that end, 320 and Up is a useful starting point for designs where mobile devices come first -- and it's free under the Apache 2.0 license.