What are some good tools or websites for comparing websites traffic, SEO, and other factors site owners might want to know about their competition.
Alexa is a website on which you can compare different websites / domains. The data of alexa isn't the best but it could be interesting for website owners anyhow.
Majestic SEO is a tool which could be interesting for you, too. It let's you compare domains, too.
I hope I could help you :)
Alexa is good for larger websites and for directionally accurate data. IE, it can help guide you if site A is bigger than site b.
Also, if available, you can use the google doubleclick ad planning tool to compare site traffic. However, the sites will have to make their sites available to google, but its free so worth a shot.
You can use a paid service like http://www.compete.com/, but i have never used them personally.
If you want to just compare domain registration companies for prices and features, I would use something like http://smallbusiness-domain.com .