Ask HN: Which tools do you use to optimize your webapps?

I'm trying to always keep size of images I publish on the web at a minimum. I have been using the smush.it service a lot, but after Y! took over the service, it hasn't been working too well -- for example I can't smush images stored locally any more.

Also, minifying and merging JS and CSS is a good trick to keep loading times down.

Now I'm interested to know -- which webapps, tools or tricks do you use to optimize your web content?