One of my most-used sites: http://dictionary.reference.com/
I use the thesaurus more than the dictionary, but they both get a work out when I'm blogging.
There are also word games (which I haven't tried yet) and a daily crossword puzzle (which I finished today, but probably won't have time for on most days.)
I feel that IF I am going to share my thoughts, I need to do so in the best manner possible. This site is one of the tools I use to do just that.