Extra awesome things that Calibre can do:

Calibre can manage multiple libraries, and you can swap between them whenever - for example, I've got a library for my kid, another for my spouse, another one for me, another one for my comics, that kinda thing. Click the Libraries icon in the top big-icon area (looks like four books together on a shelf) and use Switch/Create Library to make a new library (make sure to check the “Create an empty library at the new location” button), and then you can switch between libraries with the “Quick switch” option under the Libraries button.

I found it especially helpful to make a library called something like “Temporary Conversion Directory” for books that I'm in the process of converting or fiddling around with. Once I'm done, I right-click on the books (or select all the books first with shift-click) and use the “Copy to Library” feature to send them off to the libraries that they actually belong in. Using the “Delete after Copy” function is functionally identical to moving the books rather than copying them (it's just a poorly-named option).

FanFicFare is a fantastic plugin that can go get stories from a HUGE range of websites (and not just fanfic - here's a list) and turn them into cosy ereader-ready epubs with just a couple of clicks. Install it via Calibre's plugin manager (Preferences → Advanced → Plugins). Then you can navigate via web browser to a story you want, copy its URL (keyboard-fast: F6, then CTRL+C), paste it (CTRL+V) into the FanFicFare plugin, and it'll go get the book for you as an epub.

It also works with stories-in-progress, and can check for updates from the author (by watching the webpage for changes). Pretty neat!

(again, don't forget to hit Resize Cover before sending it to the ereader; no sense wasting space!)