I believe bootstrap will be adding a responsive grid with bootstrap 2.0, currently being developed in the 2.0-wip branch.

http://bootstrap.io/Demo/Scaffolding

Yep, responsiveness is the main feature of 2.0. Arguably not as responsive as other frameworks though as it doesn't use percentage widths to gradually scale columns. For a given width range, the column widths are fixed.

It isn't as fully featured, but is a great, simple alternative that works well on mobile also.