QuietThyme is a cross between your average cloud storage provider and the Calibre2OPDS trick which you can use to create your own web based ebook server (more on this in another post). It supports many of the features of calibre and other library apps, including the ability to search, sort, and edit the title, author, description, and other metadata of the ebooks you have uploaded.
Right now you can only upload Epub ebooks, but that will be expanded to include Mobi and MSReader fairly soon (what, no PDF?). The ebooks of course have to be either DRM-free or already configured for your device; QuietThyme is not responsible for the DRM. While the service is in beta it is free and you are limited to only 30 ebooks per account. Prices for the paid service and limitations on the free service will be announced in mid- November.
So what's the value? Well, I'm still looking at the features offered, so I cannot speak for QuietThyme.
But one reason you might want to use the Calibre2OPDS trick is to have easy access to your ebooks from anywhere. I especially like Calibre2OPDS because of how it can integrate with certain ebook apps. The OPDS standard is supported by apps including Stanza, Moon+ Reader, and more. It's an RSS-like data stream that lets you sort through and download ebooks with a minimum of hassle. If you've ever downloaded ebooks from inside Stanza or another app then you have very likely used this standard without realizing it.