The Office User Assistance team has done an awesome job of gathering and producing tips & tricks that can help your users learn how to perform both easy and complex tasks on their Office Tips & Tricks Web site. These come in the form of many helpful videos as well as written instructions. You can even sign up to have Office tips delivered by e-mail monthly here (note--this page directs you to the sign-up page, where you'll need to register in order to receive the e-mails).
Here are a few of my favorites--at least ones I've used recently because...well, let's just say sometimes I'm a little bit Excel-challenged:
It's definitely worth perusing this site to find videos that you think might be useful to link to from training sites or Web pages that you create for your users.
- Andrea Weiss