One of the more interesting new web parts in MOSS 2007 is the Site Aggregator, found on the My Site’s listing of web parts.
In theory, this one web part lets you have content on your My Site from many other web pages with its own tab bar. Basically this is an improvement over the standard Page Viewer web part which only lets you view one web page per web part.

For some reason though, the Site Aggregator has some serious flaws that seem fairly basic. The primary problem is that by default the Site Aggregator adds “/_layouts/MyInfo.aspx?ShowTasks=0&AccountName=youraccountname” to the end of every URL you put in. This is helpful if all you want to do is view other SharePoint pages, as it causes those pages to just show you your info, and not waste space in a web part with the site navigation… but if you want to view an outside web page it causes that URL to break. Try adding a New Site Tab for http://www.google.com to the Site Aggregator and it tries to take you to http://www.google.com/_layouts/MyInfo.aspx, which of course is not found.

You can improve the situation somewhat by choosing “Modify Shared Web Part” from the drop down arrow in the top right corner of the web part. This takes you into Edit Mode and gives you some settings for the Site Aggregator. Navigate to “URL” and you’ll see a field titled “URL to append to site” which is set to “/_layouts/MyInfo.aspx” by default. You would think that if you delete this, and click OK, that you would fix the problem. While that does remove the “/_layouts/MyInfo.aspx” ending to the URL, it does nothing to change the “/?ShowTasks=0&AccountName=youraccountname” addition to the URL… and “youraccountname” is actually your account name, likely in the format of domain\username.

This addition to a URL still causes many web pages to fail. Worse, it is sending out your account name to every site you add to the Site Aggregator, which is a serious security issue.

The Site Aggregator also fails to allow you to Edit a URL, or rearrange the order of your sites. If you need to make a change to a site, you have to delete it and then re-add it, placing it at the head of your list of sites. If you liked your old ordering, you’ll have to delte all of the sites and start over.

These problems make the Site Aggregator unusable for viewing non-SharePoint content from your My Site, but there is no other option. The Page Viewer web part is no longer available under the list of parts for your My Site. It was there in SharePoint 2003, and it is still available for other sites in SharePoint 2007, just not your My Site.
UPDATE: the Page Viewer web part is available, but it appears you must turn on the Publishing Feature for the site in order to access this web part, or by creating a custom feature to active it.