Back in the old days of 1.6 and its server mods, you could precache (download) any files onto anyone's PC without having to make a res file for every map. Infact, you didn't need any res files and it wasn't even map specific. All you needed was a precache_files.txt or whatever the plugin had it as, and it would do the work for you. Anyone who joined on any map would get the files. I'm sure you all know this, but I'm just restating the obvious incase someone doesn't know. This would be incredibly useful in Source, and I'm surprised it wasn't one of the earlier plugins made. Just think of the possibilities. Source custom sound plugins would be a hell of a lot easier to set up. So, is anyone up to the challenge?
Here's an example of one I used to use for AMXModX:
http://forums.alliedmods.net/showthread.php?t=700