The reason for usability testing on websites is well known, isn't it? It's to get the site seen through the eyes of someone who is not familiar with it, and to see what they make of it.
I think anyone could do a lot worse than read a great book on the subject. It's Don't Make me Think by Steve Krug.
If you want to dip your toe in the water, you can do usability tests at Five Second Tests and Feedback Roulette.
You can have your site looked at by other visitors and you give back to the community by testing other people's sites.
Just like it says in the name, Five Second Tests show you the site for five seconds and then the site disappears from view.
And it has taught me a big lesson.
The lesson is that if I don't know in an instant what the site is about, then those five seconds go by in a flash.
I spend those five seconds trying figure it out what the site is about and I just don't 'see' anything else on the page.