A service for competitors research, shows organic and Ads keywords for any site or domain.

SEMrush seem to offer a lot of their tools free, possibly with limitations. So far i have found the following

• Main Search Tool - You can search by Domain / Keyword / URL, and select which region your results are from (ie USA or  UK)
• Position Tracking
• Domain Vs Domain - Comparison of domains by keywords
• Charts
• Keyword Difficulty
• Site Audit (you need a Free account)

SEO & SEM Pros

SEMrush is created by SEO/SEM professionals for SEO/SEM professionals. We have the knowledge, expertise, and data to help you take your projects to the next level. We collect massive amounts of SERP data for more than 106 million keywords and 71 million domains, including: AdWords ad copies and positions, organic positions for domains and landing URLs, search volumes, CPC, competition, number of results, and so much more. We also provide accurate, customized data with quick turn-around times for your special projects.

Marketing Pros

Marketing professionals around the country and around the world utilize SEMrush as a part of their daily routine. From researching how products are advertised to finding the best way to phrase advertising text - SEMrush is the perfect companion to a well-run marketing department. Search trends, competition, and dollars spent by your competition are among the hallmarks of taking the guesswork out of any marketing effort. SEMrush is here to be your trusted resource.

SEMrush also produce a firefox extension called SEOQuake.