I generally use hosts file(ie blacklist), but been experimenting with writing your own proxy scripts (ie whitelist).

I really like the whitelist idea and proxy scripts are dead simple to write(http://en.wikipedia.org/wiki/Proxy_auto-config).

The problem is that even a site like Stack Overflow loads its CSS and JS libs from other non SO locations such as CDNs.

So you end up spending a bit of time setting up each whitelisted site which of course is time spent procrastinating.