When we were developing fotb.me we ensured the app would run offline using the fancy, new HTML5 app cache API. It can be a pain to create the cache manifest file by hand, this is obviously something that should be automated.
I looked around and found a plugin for Coda and a bundle for TextMate and TextWrangler which are good, but they don't allow me to ignore certain files and only add certain filetypes which were two things I needed. Not wanting to have to remember to edit the generated files manually I rolled our own.
CacheManifestGenerator is a simple Ruby script that traverses any folder recursively and makes a note of each file. You can add specific files to be ignored and the specific file extensions to look for. Very, very simple yet effective.
It also adds a comment with the time of creation. This is enough to trigger browsers to check the manifest contents as the file itself is different.
Oh and btw, Ruby is fun!
John Allsopp reminded me of manifestR, an online tool that creates a manifest for any page you are visiting via a bookmarklet. Very cool stuff, but it didn't work for us as the site has to be online, not be password protected (something we were avoiding pre-launch) and also the ignore files bit I mentioned above. However if you don't need those things then its definitely the easiest way of generating a manifest automatically.