I hadn't realised that the founders of blog search engine Sphere are the same people that built the now-defunct Waypath, which means that the extremely useful Waypath It! bookmarklet is reborn as Sphere It! This takes the textual content of the page you're looking at and uses that to run a query and find other pages with similar content.
It's probably best to change the date option to search posts from the last 4 months, which is a bit slower but produces more relevant results.