Twitter Bootstrap - "Sleek, intuitive, and powerful mobile first front-end framework for faster and easier web development."
Theming Bootstrap sites:
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Bootswatch - An open source repository of free themes for Twitter Bootstrap.
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Wrap Bootstrap - "a marketplace for premium Bootstrap themes and templates."
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StyleBootstrap.info - "Create unique web design with Twitter Bootstrap. Kw: themes, templates, styles. generator. customize."
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Customizing Bootstrap - An overview of doing customizations of Twitter's Bootstrap, relavent to v2.x