If you join, you can create a running vocabulary study sheet for the books or text that you are reading. (Those of you who are preparing for the Advanced Placement Caesar and Vergil exam may find this feature particularly helpful! A number of books from De Bello Gallico (no word yet if that's the selected text, though) and Vergil's Aeneid have already been processed on the site.)
Do be sure to take a look at Laura Gibb's helpful notes for using NoDictionaries.com first. She clearly explains a lot of the features in detail.
Other helpful dictionary reference tools to help save you time and read more Latin include:
The Perseus Project: Charlton T. Lewis and and Charles Short's A Latin Dictionary. This authoritative resource provides very detailed definitions and citations from Classical Latin authors showing each word used in context. Also available at the Perseus Project is Charlton T. Lewis' An Elementary Latin Dictionary, which is ironically shorter than the Lewis and Short version.
Available on the iPhone and iPod Touch App Store are two Latin dictionaries for download: Latin Dictionary and Lexidium. These are easily found if you have an iPhone or iPod Touch. Simply click on the App Store icon and do a keyword search for the titles.