If you want to include information from a different domain within a web page, you might use an iframe. The trouble is that the contents of the iframe could vary in size and, because it's cross domain, the parent can't access the iframe contents to determine how big they are; the iframe is also unable to send messages to the parent.
Now that Firefox 3 (>= beta 3) supports window.postMessage(), as do the latest WebKit nightlies and Opera 9 (though Opera uses document.postMessage()), it's possible for the iframe to determine the size of its contents and send that to the parent window, which can then resize the iframe accordingly:
In the iframe (which has an element with an id of "foo" enclosing the main content):
var target = parent.postMessage ? parent : (parent.document.postMessage ? parent.document : undefined);
if (typeof target != "undefined" && document.body.scrollHeight)
window.addEventListener("load", postSize, false);
In the parent document (which contains the iframe with an id of "bar"):
Update: added wildcard origin parameter, as postMessage changed slightly.
if (e.origin === "http://www.example.com") // for security: set this to the domain of the iframe - use e.uri if you need more specificity
document.getElementById("bar").style.height = e.data + "px";
window.addEventListener("message", receiveSize, false);