I always keep several pocket dictionaries in the classroom, and we use them constantly. Since most of my writing (and much of my reading) is done online now, and the same is true for students, here are a few of my favorite out-of-pocket dictionary recommendations:
My most faithful companion for several years now is WordWeb, which offers a free download of a pc dictionary. Highlight any word and click on the icon in the task tray, and the definition comes up, along with synonyms, antonyms, and other very useful information. Sometimes I just need help with spelling, and if I've come anywhere close to the correct word, WordWeb will suggest the correct spelling.
In the classroom, we need to be able to share definitions, and that's where the next two web dictionaries come in handy. Definr bills itself at an "incredibly fast dictionary," and it is that. A kid-friendly mascot meeps! useful definition information in an instant. You can check out the word-of-the-day (today's word is 'cloture'), and even have it sent as a feed to your homepage or feedreader. What I especially like about Definr is that it uses the words in a sentence. A nice feature for Firefox users is a bookmark toolbar button, allowing for one-click lookup.
Ninjawords describes itself as "a really fast dictionary, fast like a ninja" and I'll vouch for its speed, though I don't know many ninjas. It provides all the information a useful dictionary should, with one outstanding extra feature: it maintains a history of the words you've looked up. That might be useful when you are gearing up for a vocabulary quiz.
Looking up words isn't what it used to be: now, it's 'really, incredibly' fast!