My current tools include YSlow, Fiddler and the new PageSpeed.
My questions is: is there a better tool for finding out exactly WHAT the browser is caching, WHEN it's caching it, and WHY other assets are not caching.
A brain-dead simple explanation of HTTP headers, vis-a-vis caching would also be useful. The more gotchas the better, such as "Greenwich Mean Time" is what the browser relies on, etc.
At the risk of self-promotion I've written Supercharging Javascript in PHP and Supercharging CSS in PHP, which deal specifically with caching (on the client and the server) including appropriate HTTP headers like far-futures Expires and ETag to minimize external HTTP requests.
the browser doesn't have a concept of local time. It simply sends the servers own last-modified date back at it as the value of if-modified-since and lets the server decide how much time has passed.
I just use the webdeveloper toolbar addon for firefox and check the 'response headers' manually. It's usually pretty obvious when something is/isn't being cache since the browser renders incrementally. There is a way to force a fake connection speed as well, but I can't remember how. Slow the rate to 14.4k and you'll really see where your caching is working.
www.fiddler2.com/redir/?id=httpperf explains how WinINET (the HTTP stack behind Internet Explorer) performs caching. See also http://www.fiddler2.com/fiddler/Perf/AboutVary.asp