Through the course of my day marketing our client's websites, there are tools I use time and time again. Seriously, a day doesn't go by that I don't use all the websites listed below at least once! They're free (at least on some level) and offer lots of information, and there's absolutely no reason you, as a web marketer or owner of a website, shouldn't be using them as well.
I've been using Compete.com since day one. It's an amazing tool for getting estimates on the traffic of a website. They have a huge base of U.S. users that get counted as visitors to your website, should they land on it. It's only U.S. traffic for now, and needs to be taken with a grain of salt since it's based on their users. That said, it's the most accurate website traffic estimation tool I've ever used.
Open Site Explorer
The guys over at SEOMoz have come up with an amazing tool in the Open Site Explorer (OSE). My colleague and friend Mike P. likes to refer to it as "Yahoo Site Explorer on steroids." Once you check it out, you'll see the guy makes a good point. It gives you all important metrics of your backlinks, including the number of root domains and the URL of The big differentiator between OSE and Yahoo Site Explorer is that OSE only shows the top 25 links from any given root domain. Yahoo will show you thousands of links from the same URL on different pages, which isn't nearly as effective or helpful.
SpyFu is the ultimate PPC competitive research tool. It will show you what keywords your competitors are advertising on, their average daily spend, their average cost per click and more. If only it showed conversion data...
Google Keyword Tool
The Google Keyword Tool is the most accurate estimate of traffic out there. It's ideal for keyword research when thinking about what terms to go after for SEO, what keywords to focus your PPC on and even for good 'ol research into a new niche.
Michael Wong's AdWords Wrapper is a huge time saver when creating or expanding AdWords campaigns. It will automatically create different match types of your keywords lists and let you copy and paste them out into your account.
FireBug + Web Developer Toolbar
These two Firefox add-ons are geared primarily toward web developers, but I use them to my advantage as a marketer as well. I'm constantly using them to quickly see if links are follow/no-follow and to see if client and/or prospects' websites have all their on-page SEO elements in place. Get FireBug and the Web Developer Toolbar today.