A few years back I mentioned being surprised that more developers weren't jumping on the Mozilla framework RIA bandwagon. The same technologies (HTML, JavaScript, Ajax, DOM, XBL, XPCOM, XUL, RDF, RSS, Chrome...) have been a boon when it comes to the plethora of Firefoxextensions, but very few full-blown applications ever surfaced. MAB was one I always pointed out as a good example. I felt there was huge potential there, but for whatever reason it never took off (MAB has been discontinued due to changes in the Amazon Web Services backend).
Hosted Applications
In the past year, it seems the latest rage in JavaScript development are so-called hosted JavaScript application frameworks. The general idea being you use the same language both in the client and on the server-side. On the server-side you have access to SQLite, MySQL and other data sources such as XML stored on disk (or generated), and other resources—just like you're used to with languages like PHP, Python, Perl, or Ruby—and on the client-side you have access to the browser DOM, CSS, and so on, then glue it all together with XHR/JSON. Most of the offerings in this genre include browser-based IDEs for rapid application development, and you can fork off someone else's code to quickly generate a morphed version using your own concept. Though the technologies are open-source, most of these companies monetize their investment by hosting (and charging consumers for) the application, much like the cloud computing models (e.g. EC2, S3) that are becoming increasingly popular.
Although there are many similar RIA frameworks, some of them open-source and some of them not
(see below), I'm going to concentrate on the two that I found most interesting—both of which are open-source.
Jaxer
Jaxer is an open-source, client-server Ajax framework built from Mozilla's SpiderMonkey JavaScript engine. With it you can access data using built-in SQLite, or externally through a DB connection to MySQL. I'm not sure whether they have or are planning on supporting other RDBMSs such as PostgreSQL. I would think a database abstraction layer would be handy. Naturally you also have file I/O and access to everything on the server end. On the client-side you have everything the browser has to offer: markup, DOM, CSS, and finally asynchronous XHR using JSON, or whatever you prefer, for sending data back in your response to the client.
A really slick feature of Jaxer is the ability to code JavaScript that runs on both sides of the equation. In other words, things like form input validation in the same language so you don't have to repeat the same functionality. Also of interest is their choice of going with SpiderMonkey, which supports JavaScript through version 1.8. Think you'll miss your favorite JavaScript library? Think again, you can easily access jQuery, prototype, and the many effects libraries. Does the SOP restriction on Ajax between client and server frustrate you? Not any more, since the server-side code can access data from anywhere before transporting it back to the client. Only you can trust it. Because you know what you're doing, right?
I have not seen any benchmarking done on the system, but when TraceMonkey rolls out performance should go way up. In case you can't tell, I'm pretty excited about this framework and the technology behind it.
Acre
Acre is a similar offering from the folks at Freebase. Just out of developer preview status is this new integrated Web application and hosting environment. Using a simple JavaScript templating language and their IDE (the app editor)
you can create applications using templates, scripts, queries and static (e.g. CSS) files.
Unlike Jaxer, I was not able to find much in the way of details on how they implemented the system. However, to see what's possible I recommend taking a look at FMDB. My impression thus far is it has potential, but they need to beef up the documentation and offer more examples and tutorials.
Similar RIA Frameworks
If these guys are the new kids on the block, they certainly aren't alone. Offerings in the RIA arena have been around for a while. However, most of those listed below to not fit into the hosted application model.
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A few years back I mentioned being surprised that more developers weren't jumping on the 
