Did you know you can branch your own personal version of many of the sites
hosted at Branchable? Look for the "Branchable" link on sites, which leads
to a form for branching. Or, use the Control Panel
to branch your own sites. I think being able to branch a site is a great
feature. That's why it's called "Branchable". 
--Joey
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You can make branches of your own site to develop things outside from public view, and then use Git to merge your changes back into the original site when ready. Or you can branch someone else's site to make improvements, or to customize it into something of your own.
You can always branch sites that you own or administer. Other sites can be branched only if their owner allows it.
Another way to branch a site is to use Git to check it out of Branchable, and use Ikiwiki to put it up elsewhere.
A related thing you can do is to put a branch of your site on your laptop, so it's always available. You can then push changes from it to the public site at Branchable. Ikiwiki has documentation about how to do that at http://ikiwiki.info/tips/laptop_wiki_with_git/.