http://blogoscoped.com/credits/

How do you keep track of this? Tags? How often does this update?

It's automatically updated and analyzes all the blog posts: it always looks for the "via" line and then checks which name or site title is in there. While there are some parameters that allow for fuzziness – e.g. both strings "Thanks Brinke" and "Via Brinke Guthrie" would be counted for the same person – it's not 100% precise (and currently doesn't handle multi-person thankings :)). The page is meant to be a bit like an automatic blog roll or "things I'm reading" page.

There are two lists – the top one looks for occurrences in the last year, and the bottom for all-time "via" lines. For instance, you'll notice that "Digg" comes in an all-time second as a source, but that I only referred to it twice the last year.