Reading this post from Stephen Sharrock reminded me about something I often overlook, the phenomenally powerful DHTML behaviours which IE supports using HTC files. The WebService behaviour for instance lets you pull information into your client from aribtrary web services, even without using HTCs, tools like xLoadTree provide the ability to load data straight into the client without forcing a postback.
Before anyone comments, I know these are IE only - to be honest I don't care, when only 3% of users use anything other than IE, it seems a shame to ignore the functional improvements these kind of tools provide.
Now, Mozilla does support it's own version using XBL and XUL - hopefully some genius will work out a way for mozilla to support full HTC as well...