There are a number of possible replacements but none of them could be called "good" in that they cannot compare with iWeb's ease of use, intuitiveness and price.
Sandvox is probably the nearest to iWeb in the way in which it works but doesn't quite cut it.
RapidWeaver is very expensive by the time you consider the cost of all the add ons.
Freeway is antiquated and clunky.
Quick 'n Easy Web Builder is the nearest in price.
And so on...
There are also quit a few alternatives if you want to step up to the plate and learn some code. There's a link to a download listing some of the alternatives on this page...
I wouldn't be surprised if an application very similar to iWeb arrives on the market soon. In the meantime you can keep iWeb running under the newer versions of OS X as shown here...