We update our game frequently, but our users' browsers have cached old versions. Is there any way to tell the browsers that they should download the `.unity3d` file anew? It would be okay to turn off browser cache completely since our game is teeny.
Adding a query string to the .unity3d file's URL will not always work, for whichever reason - browser caches are really fickle beasts. The way I solved this problem has been to always append the build number to the published .unity3d file, like MyGame436.unity3d. That way, no matter what strange caching algorithm the user's browser uses, it'll redownload the game for sure, since it perceives it as a completely new file.
I'm doing the renaming automatically through a batch file that I run every time I want to publish a new version of the game to the server. It reads the current SVN version of the file through subwcrev (let's say it's 436), then copies MyGame.unity3d as MyGame436.unity3d and changes the .html file to reference MyGame436.unity3d.