I must admit, I use the dictionary built into OS X a lot. Whether to choose the right word or to understand one, it's a handy tool. But as always, internet-land has some tricks. You won't always find the right meaning of a word in common parlance online in a regular dictionary-
For example, this is the entry for 'leet' in the Oxford Dictionary that the Mac app uses:
Urbandictionary.com has been the go-to guy for this problem, with questionable results.
The great thing about urbandictionary.com is that every-day interweb users can post their own definition of any word. That means that H0m13_d4wg_1998 can tell me exactly what he means when he says 'leet', in his own words.
The problem is that definitions may be offensive, irrelevant or just plain wrong (look at the list of definitions for leet as an example).
Macmillan Dictionary has an answer: Open Dictionary. Basically, the concept is still one of user-submitted definitions, but now instead of being voted up and down the page by other users, the good people at Macmillan do the editing, adding the definition to their mainstream dictionary when appropriate.
Open Dictionary by Macmillan Dictionary via LifeHacker