A quick, clickable classification
I came across this LibraryThing shortcut to generating an approximate DDC-like number for specific titles.
You simply click on the boxes that correspond to category of the book in front of you and it builds a DDC-like number. (They use "Melvil" because "Dewey" is copyrighted.)
Then it generates a list of titles in LibraryThing that have that number, and some categories at the bottom of the page that probably overlap ("Far friends"). You can also see a bit of a word cloud/tag cloud to the right that gives you a sense of what topics the real DDC number would likely address. (If you have a LT account, you can even see which of your own books fit the category.)
Not as accurate as some other tools, but rather a "quick and dirty" guide.
(It's also fun to click through and see what titles come up.)