FWIW: Rails also makes RESTful APIs very easy. (And Prototype goes along with that, in the way it handles AJAX request returns.)
While Rails is very well integrated with Rails, they're not very tightly integrated. I was worried about that, but it turns out it's pretty easy to use other JavaScript frameworks with Rails; Rails doesn't give you any crap if you just drop in MooTools or jQuery instead.
(Plus, of course, jQuery does a bang-up job of cooperating with other JS frameworks. I was pretty impressed when I saw how easy it is to use jQuery alongside anything else.)
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While Rails is very well integrated with Rails, they're not very tightly integrated. I was worried about that, but it turns out it's pretty easy to use other JavaScript frameworks with Rails; Rails doesn't give you any crap if you just drop in MooTools or jQuery instead.
(Plus, of course, jQuery does a bang-up job of cooperating with other JS frameworks. I was pretty impressed when I saw how easy it is to use jQuery alongside anything else.)