One of the things that's always annoyed me about web app development is how klutzy it is to try to persist data locally (offline, say) from a script running in the browser. The cookie mechanism is just so, so . . . annoying.
But it turns out, help is on the way. Actually, Firefox has had a useful persistence mechanism (something more useful than cookies, at least) since Firefox 2, in the so-called DOM Storage API. Internet Explorer prior to version 8 also had a a similar feature called "userData behavior" that allows you to persist data across multiple browser sessions. But it looks like the whole browser world (even Google Chrome) will eventually move to the DOM Storage API, if for no other reason than it is getting official W3C blessing.
The spec in question is definitely a work-in-progress, although the key/value-pair implementation has (as I say) been a part of Firefox and Spidermonkey for quite some time. The part that's still being worked out is the structured-storage part -- the part where you can use some flavor of SQL to solve your CRUD-and-query needs in a businesslike manner.
Why do we need a W3C structured-storage DOM API when there are already such things as Gears LocalServer, Dojo OfflineRest, etc. (not to mention the HTML 5 ApplicationCache mechanism)? A good answer is given here by Nikunj R. Mehta, who heads up Oracle's related BITSY project.
Most of the current debate around how structured storage should be handled in the DOM Storage API revolves around whether or not to use SQLite (or SQL at all). The effort is presently heading toward SQLite, but Vladimir Vukićević has done an eloquent job of presenting the downside(s) to that approach. The argument against SQL is that AJAX programmers shouldn't have to be bothered to learn and use something as heavy and obtuse as SQL just to do persistence, when there are more script-friendly ways to do things (using an approach something like that of, say, CouchDB). As much as I tend to sympathize with that argument, I think the right thing to do is stay with industry standards around structured data storage, and SQL is a pretty universal standard. Of course, then people will argue that "SQL" isn't just one thing, there are many dialects, etc. Which is true, and a good example is SQLite, which the current DOM Storage effort is centering on. I happen to agree with Vladimir Vukićević (and others) who say that SQLite is just too limiting and quirky a dialect to be using in this situation.
I bring all this up not to argue for a particular solution, however, so much as to just make you aware of some of what's going on, and the current status of things. Explore the foregoing links if you want to get involved. If having a common data-persistence and offline/online HTTP-cache API that will work uniformly across browsers means anything to you, maybe you should join the discussion (or at least monitor it).
Getting beyond cookies is essential at this point; the future of web-app development is at stake and we can't wait any longer to nail this one. (We also can't do goofy things like rely on hidden Flash applets to do our heavy lifting for us.) It's time to have a real, standardized, browser-native set of persistence and caching APIs. There's not a moment to lose.