I recently read this post by Francois Zaninotto (recently an ex-core symfony developer) about modifying symfony to make it lightweight enough for ajax calls. As he puts it:

That’s when the idea of a “small symfony” comes. Wouldn’t it be great if you could get access to the model layer, the configuration, the autoloading, the user object, the helpers, and keep a MVC separation, without initializing the whole framework?

As symfony stands, it’s sometimes just a tad too bloated for the kinds of really fast responses needed to make ajax applications work. Francois came up with this solution, which is to cut out the call to sfController::dispatch()

But you still get enough to make use of things like the basic MVC design, autoloading, and helpers. I still haven’t tried it myself, but it seems like a really nice idea. Just a shame he’s no longer in the core team…