For those of you who don't already know, XSLT is a techno-thingy that lets you make changes to an XML document. In lay terms, because an RSS or Atom feed is written in XML, you can run it through an XSLT and modify it.
Sam Ruby writes here about several XSLT's he's written that fixes some minor problems with one feed and does filtering on another. Tero Ykspetäjä also submitted an XSLT that reverses the order of items in a feed, to work around the problem of some feeds publishing their items backwards
Awasu is one of the few feed readers that let you apply an XSLT to incoming feeds which is quite a powerful feature. As you can see above, you can do some pretty cool things with it