Just thought I'd pass this on.. I recently was looking into where all the space on my OS X partition had gone.
I looked around and noticed that my ~/Library/Preferences/Explorer folder was around 400 MB (my cache is set to 5 MB). For some reason, the Download Cache was a 390 MB WAFF document. I trashed it, quit and restarted IE, which generated another Download Cache (only 4 KB), while keeping my download history intact.
I also checked my little brother's computer for the same thing, and he also had a relatively large file (~75 MB) there. We're both running 10.1.2, IE v 5.1.
[Editor's note: With the proliferation of browsers, cache can definitely take up a fair bit of drive space in a hurry. If you use multiple browsers, you should check each occasionally to see how much cache space they're each taking up.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020116023308813