There are a number of new features in the recently-released Rails 2.2, including ETags. HTTP ETags (or "entity tags") are a way of uniquely identifying content sent by your web server. Rails 2.2 adds specific support for ETags and checking the last modified date to help you save a little bandwidth.
It works like this: clients save the ETag when they request a page and, if they ever request the page again, they send the ETag along with it. The server can look at the ETag to determine if the content has changed since the client last requested it. If it hasn't changed, the server can send an empty HTTP body, saving quite a bit of bandwidth. For pages that users refresh often, such as the main page of a frequently visited Website, this can be a big bonus.