Web Workers fix that, in effect, but this post is about a library designed to combine one kind of multithread-ish-ness with another: arbor.js, a graph visualization library using Web Workers and jQuery.
One of the neat things about graph visualization, as opposed to (say) text-streams (or tables), is that a visible graph lets the user's eyes scan over multiple things at once, in a path the user chooses (though with some paths suggested by connecting lines) -- thus giving the user's massively parallel brain access to all kinds of information at once.
And arbor.js does this extremely well, taking full advantage of cutting-edge web technology to produce extremely smooth visuals, with an extremely straightforward API. (That API reference page is coded pretty slickly too.)
Here's how one sample project looks:
Actually I'm a sucker for any graph (or tree), so this kind of thing especially lights up my brain.