I’ve been using Google Reader for quite some time now, but it was only today that I noticed that I can search within a single feed. The drop down menu near the search box at the top contains choices like “All items”, “Starred items”, and “Shared items”, which are followed by the tags. But if you scroll further down, after the tags, there is a list of all your subscriptions. You can pick any feed that you are subscribed to and search within it.
I’m not sure if this is a recently added feature or if it was there for ever, but it’s priceless. I was missing out on it, because I use way too many tags and, apparently, never scrolled down deep enough.