This post is a follow up to the one I wrote a couple weeks back. That one covered 5 useful tricks for calibre. If you've used the app then you probably there's far more than 9 calibre features, so I will probably add another post in this series.
Read the News
Calibre is known as an ebook library tool but it does a lot more. One featured loved by a number of dedicated readers is fetching the news. This lets you set a schedule for calibre to scan and download the contents of news sites and blogs. Calibre has to be running and your computer turned on, but the nice part is that calibre comes pre-configured with over 1300 news sources from all over the globe.
In my previous calibre post I showed a few plugins which added extra abilities (like removing DRM). Here is another. Wiki Reader is a plugin which lets you grab the contents of one or more Wikipedia articles and build them into an ebook.
You can find the plugin over on MobileRead.
Import eBooks from an eReader or Tablet
One feature that I keep forgetting, but is terribly useful, is how calibre can find ebooks on your tablet or ereader and add them to your calibre library ebook collection. This might not happen to you, but I often download ebooks from free ebook sites (Manybooks, Feedbooks, author's websites, etc) to whichever tablet I am using. I don't always remember to get a copy later to add to calibre, but that is largely because I don't have to. All I need to do is plug in my tablet via USB and calibre will copy the ebook files off of it.
If you have a well maintained calibre library, and you have an account at Dropbox, you might want to consider combining them in a clever way. While you can always upload your ebooks and download them manually, there is a better option.
Calibre2OPDS is an app for Windows, Linux, and OSX which build an OPDS catalog based on the contents of your calibre ebook library. Upload your ebook library to Dropbox, add the OPDS catalog generated by Calibre2OPDS, and assuming you follow all the steps you will be able to download your ebooks from inside reading apps like Stanza, Moon+ Reader, and others.
So what's your favorite calibre feature?