Here’s a great presentation on usability by Steve Krug, the foremost expert on the subject by far.
The focus of this presentation is on the importance of user testing. Most of the time, user testing never happens; or, when it does, it is done in way more detail that it needs to be, with scientific analysis of the results and a long report, costing about $10,000.
However, you can be just as effective with user testing by just testing three users, watching how they use your site while they think out loud (that’s the key). With just three users you will identify far more problems than you have the resources to fix anyway. Identify the top three, address those, and you will have made a significant impact.
Last: don’t do user testing as an add-on at the end. You need to do it throughout the process. If you do three rounds throughout the development of your product (website or anything else–I think all products should be user tested, even building layouts), fixing the top three problems identified after each time, that would be huge.
If you want to save time, the best way to watch his lecture is actually to click through the slides at the bottom. And, for more detail on what makes for good usability on websites, see his excellent book:
(HT: Matt Heerema)