Many different word processing programs offer the ability to display a word count for your document. Here is a compilation of instructions on using word counts for different versions of Microsoft Word and several web applications. Click on the link to see the instructions for your software:
MAC OSWord for Mac 2011
Pages for Mac
WEB APPLICATIONSGoogle Docs
Don't see your software listed here, or want an easy online solution? Check out Word Counter, an online word counting web app.