Seaside is one approach, there is also Aida-Web. Those are implemented in pharo.
with Amber you can use also node.js if you want. And any server side node.js library you can get your hands on. Amber can use any js library.
You could even mix amber, pharo , node.js and seaside / aida-web .Sky is the limit. Its up to you which specific approach you will choose. You definitely have loads of options.
You forgot Iliad: automagic AJAX, plus clean and short codebase you can read and understand in under a day (vs. thousands of packages in Seaside).
There is of course a downside: a really tiny community... took me a while to get used to the Iliad way, but I'm not going back unless Seaside shrinks to 1% of its current size and makes AJAX integration as easy as Iliad does :)
Anyway, I guess the future is not about server-side frameworks, but something like Amber + some sort of Pharo REST service (kinda like Seaside-REST) + some sort of magic persistence (kinda like Voyage).
I had a shot at Aida-Web, not my cup of tea. Does the job though. Amber is another beast, and is a very good option for single page web apps. Can do proxying of JavaScript objects, so you can harness whatever you like in your code and use the JS API the Smalltalk way. Which is kind of cool. With the Helios IDE, it is a killer. Needs a while to master but worth the time.
with Amber you can use also node.js if you want. And any server side node.js library you can get your hands on. Amber can use any js library.
You could even mix amber, pharo , node.js and seaside / aida-web .Sky is the limit. Its up to you which specific approach you will choose. You definitely have loads of options.