I use several bits of software as I write.
I use OpenOffice.org as my word processor. It is open source and free.
I use Notepad2, a free text editing program that makes M$ Notepad look like rock and chisel.
And I use WordWeb Pro as my dictionary/thesaurus. And dude, this program is the best! I use it quite often. There is a free version but to really take advantage of all it can offer, I suggest the Pro version. The biggest change from 4.8 (which I had) to the 5.1 (which I just upgraded to) is the web browser look-ups.
With my purchase, came a Wikipedia plug-in. This means that when I look up a word, it not only checks the dictionaries on your computer, but also Wikipedia, Wiktionary, and WordWeb Online.
Another new thing is its ability to connect to other dictionary programs you may already have, such as Encarta or Webster. This would be great for those more high-brow words.
So, yeah, WordWeb Pro is expensive ($49 package or $73 separate) if you get all the bells and whistles in terms of word lists (language, geographical, and “additional“). If you just want the program, that’s $29. I sprung for the entire thing to see how much I actually used any of the lists.