I am enjoying the tags in Calibre a lot. Ever since I figured out how to add cover art, it has been fun to browse my books and make lists. I have tags for source (Fictionwise, Kobo, Mobile Read etc) and for genre.
I also have a few special collections. One of them is a Canadian collection, of all-Canadian authors. The other I am calling the 'premium' collection for lack of a better word and it's basically my purchased books, but only the ones with pretty covers. So I can browse my purchased books without the amateurish covers of some of the multiformat freebies and without the list being over-taken by Star Trek books
I use the star system to show the book's status---4 stars is personal documents, 3 stars in 'on the Kindle right now' 2 stars is 'already read it in paper and have the ebook for re-reading later' and one star is 'already read.' So I can open any of my collections and at a glance see what's going on. I can sort by star and all the not-on-the-Kindle unread books go to the top and I can see what's good to add in my next sync. Once Kindle has collections, I will load every unread book on there and delete them off the Kindle as I read, like I did when I had the Sony, so I may re-assess the star ratings then.
Anyone else have any special tags or special collections they use?