Mouse Training is a UK-based company that offers training courses to companies whose staff use Microsoft Office. Following the release of Office 2010, the company has made available all of its training manuals for previous versions, totally free of charge. So if you're one of the millions of people using any of the Office 2003 or 2007 apps, and you'd like some in-depth information on how to use it properly, just head here.
Near the top of the page you'll find free downloadable Quick Guides to the Office 2010 apps (needless to say, the full manuals for Office 2010 aren't free!). Scroll down, and you'll find the free guides for Office 2003 and 2007, including Word, Excel, PowerPoint, Visio, Outlook, FrontPage, Project and Access.
These are superbly-written tutorials, produced by professional trainers. Whatever version of Office you're using, they're definitely worth a look.
NOTE: They have now added Excel 2010 (Introductory) to the free download list!