Cache_invalidation is a project mainly written in JavaScript, it's free.
Browser cache invalidation with Javascript
#
#
Invalidates browser's cache for a given page based on the last refresh.
Usage:
Include in the HEAD of your page and set the TTL
And set the TTL you want into the cache_invalidation.js file: // TTL: set your cache threshold here var ttl = 300; // seconds
The querystring.js component I got from Adam Vandenberg: http://adamv.com/dev/javascript/querystring
See also for a complete explanation on how it works and why: http://stockrt.github.com/p/browser-cache-invalidation-with-javascript-and-querystring