Today I randomly noticed on my HTC Evo a “Downloads” app which brings you directly to all of the downloaded files and attachments.
I didn’t realize there was a separate app for this, but had wondered if there was a way to return to browser downloaded files. Within Downloads, I found over 65MB of files, some of which I downloaded over a year ago. With a few taps I cleaned them all out. These are treated separately and outside of the browser cache, so if you download a lot you should periodically clean them up.