It really does not matter that iWeb is not supported. If you have it you can still use it to create and maintain your sites. It is an application and will still continue to work. Just because something is not supported it does not mean that you can't use it.
If you really want to look at alternatives, then take a look at RapidWeaver, Sandvox, Freeway Express and Pro 6 and Flux 4. There is also Quick 'n Easy Website Builder.
You then have the online website builders such as http://www.weebly.com or http://www.wix.com which are good alternatives, or you can go for the Content Management Systems (CMS's) if you already have hosting space such as http://drupal.org and http://joomla.org.
There is also a new app being developed by Rage software at present know as EasyWeb that should be released soon. However, at present you won't be able to get it or buy it separately - it will be linked to Rage hosting, so if you want this you will have to purchase hosting from Rage too to be able to use the app.
There are lots of alternatives out there and you can download free trials of most, but in the end, if you still have iWeb, then you can still use it without it being supported and it will still work. An app does not need to be supported for it to continue to work. It will stop working one of these days with future OS upgrades, but at present, it still works with both Lion and Mountain Lion.
In addition to Ethmoid's post be aware of this:
In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
To open your domain file in Lion or Mountain Lion or to switch between multiple domain files Cyclosaurus has provided us with the following script that you can make into an Applescript application with Script Editor. Open Script Editor, copy and paste the script below into Script Editor's window and save as an application.
do shell script "/usr/bin/defaults write com.apple.iWeb iWebDefaultsDocumentPath -boolean no"
tell application "iWeb" to activate
You can download an already compiled version with this link: iWeb Switch Domain.
Just launch the application, find and select the domain file in your Home/Library/Application Support/iWeb folder that you want to open and it will open with iWeb. It modifies the iWeb preference file each time it's launched so one can switch between domain files.
WARNING: iWeb Switch Domain will overwrite an existing Domain.sites2 file if you select to create a new domain in the same folder. So rename your domain files once they've been created to something other than the default name.