AppSumo has a deal going on right now for webtexttool - big fan of it so far. You can paste any URL in and it will analyze it, then tell you word count, keyword densities, synonyms to use, and more - pretty solid if you're looking for something more advanced than just word count.
However as the poster above me said, there are many free ones online - it's also built into Word, and I would assume Open Office has that feature as well.
What is WordCounter?
Apart from counting words and characters, the online editor can help you to improve word choice and writing style, and, optionally, help you to detect grammar mistakes and plagiarism. To check word count, simply place your cursor into the text box above and start typing. You'll see the number of characters and words increase or decrease as you type, delete, and edit them. You can also copy and paste text from another program over into the online editor above. The Auto-Save feature will make sure you won't lose any changes while editing, even if you leave the site and come back later.