XSLT was awesome back in the day. You could get a block of XML data from the server, and with a bit of very simple scripting, slice it, filter it, sort it, present summary or detail views, generate tables or forms, all without a server round trip. This was back in IE6 days, or even IE5 with an add-on.

We built stuff with it that amazed users, because they were so used to the "full page reload" for every change.