There’s a lot that’s wrong with iWeb, but one of the most insanely annoying things is the way it allows you to create supposedly separate web sites, but insists on uploading the content of all your sites to any server you might try to connect to.
Enter iWebSites. This neat little chunk of donationware messes (in a nice way) with the data stored in ~/Application Support/iWeb/Domains.sites. This is where iWeb stores all its stuff; site settings, pages you’ve created within the app, but not HTML you’ve exported.
Consequently it helps you manage a series of separate sites in iWeb. You can choose one as the default - the one that will open if you simply launch iWeb - and can choose which site you wish to open by just selecting it in the list and clicking “Open now in iWeb”. To use it, you have to learn to launch iWebSites first, before launching iWeb itself, but that’s a pretty shallow learning curve.