BootstrapCDN.com
Hosts the Javascript, CSS, and Image files.
Additionally hosts the Font Awesome CSS and a selection of Bootswatch themes.
Maintained by @jdorfman and @mervinej.
CDNJS.com
Hosts the JavaScript, CSS and Image files. Check the GitHub repo for more specifics.
Also hosts a ton of other JS libraries, including plenty of Bootstrap plugins/addons.
Maintained by @peterdavehello.