I haven't spent a lot of time on this one, but in browsing through it, it looks to be a fairly good reference for word usage, especially words that are frequently confused. In fact, the name of it is Confusing Words. Of course, you have to be aware that you are confused by a group of words (such as 'there,' 'their,' and 'they're') in order to look them up!
It seems that this website isn't functioning at the moment. I was on it yesterday, so I don't know if it's a temporary problem or if the site is down. A shame, because it was a nice little reference!