As someone who works with words for a living, I'm tempted to use sites like Dictionary.com. But as someone over the age of 30, I'm also quite fond of my old Merriam-Webster's, with its soft pages perfect for pressing flowers or telling me whether "flower bed" is one word or two. Which is a better bet to check spelling? (And please don't say spellcheck. I won't go there.) My dictionary is from 1998, so is it outdated?
You don't just need dictionaries--you need lots of them. With only one dictionary, you can't get in fights. And what fun is grammar without the fights?
Take our "unique" debate of a few weeks ago. I, citing Merriam-Webster's 11th Collegiate, argued (correctly) that "more unique" is a perfectly acceptable usage. A couple of Brits, meanwhile, got their knickers all in a twist, since the Oxford English Dictionary doesn't allow it. The wussy tenderfoots at American Heritage Dictionary go completely limp-wristed, saying "unique" in the comparative is legit but "informal." Pansies.
So you can find that "flowerbed" is one word, but you'll be wrong. Even as two words, "flower bed" doesn't appear in the OED or Merriam-Webster's. Pressing a bunch of flowers in your Merriam-Webster's is the only way to get a flower bed in there.
Is your 1998 dictionary outdated? Not really. Merriam-Webster's adds about 100 new words to the dictionary every year, but most of them are useless. (From last year: "chaebol," "sudoku," "snowboardcross." Bor-ing.) Your intuition about what actually relevant words are new since 1998 should be pretty good ("speed dating," "crunk," "DVR"), so you can check them online.
The OED, though, has been working since 1993 to review every word in the dictionary, publishing updates online--highly preferable to shelling out $895 for a hard copy. I'd say that's a fairly unique endeavor, but they might disagree. We'll see if they do after they hit the letter U.