For most users, iWeb keeps things simple by creating and managing a single, all-encompassing site file stored in your home directory. The downside for anyone who would like to create more than one site with iWeb, however, is that the app doesn't offer any easy way to do so; users have to drill into ~/Library/Application Support/iWeb/ and either manually move or delete the single site file in that folder, or pick up an app like Multisite for iWeb or iWebSites. Both of these apps allow you to easily create independent iWeb site files that can be published to different local folders and subsequently uploaded with external FTP apps to anywhere you chose.
Fortunately, both apps have just been updated to work with the new iWeb '08, though note that iWebSites is at a 2.2 alpha stage, as the developer isn't positive all the kinks have been worked out yet. While I haven't used either of these apps, a quick perusal of their features sets reveals that iWebSites has a unique trick up its sleeve: it can merge two different iWeb site files into one, though it apparently can take a while. Anyone have experience with these apps?
Multisite for iWeb is available from Clarkwood Software for $19.95 with a demo available, while iWebSites is offered as donationware from Cocoadrillo Software.