I have been a long time fan and user of twitterbootstrap. It's an extremely simple, yet powerful little framework for creating responsive front ends for web applications and it goes pretty far in removing a lot of the annoying things you have to do when creating user interfaces.
Having said that it's not perfect and can be a bit confusing to use if you don't know what you're doing. Typically the issue is with the style class names and naming conventions employeed.
Enter Semantic UI. Basically it does everything that bootstrap does, but does it in such a way as to remove the often obtuse vocabulary overhead one must learn in order to use bootstrap correctly.
Pretty cool if you ask me.. It's still in pretty early stages, and as such there are errors and a lot of @TODO's still in place... but it looks extremely promising.