An immensely noddy Chrome extension which blocks half of all subrequests at random.
The idea here is to test whether a page is still usable and still makes sense if it doesn't get its CSS or JS loaded. So, with this extension installed, every time a page requests some subresource, the machine flips a coin and if it comes down tails, blocks that request.
Note that this means that refreshing a page likely will cause it to fail to fetch a different set of subresources, so you can just hit refresh a few times to see differing effects.
Doesn't have any configuration or anything since it was built as a little experiment by Stuart, and you'll need to install it as an unpacked folder (or package it yourself if you like), but the results are interesting.