Keyword Discovery is a paid subscription service that offers marketers the ability to see keyword data across multiple search engines. They maintain one of the largest databases of searches, (nearly 38 billion) which are compiled from over 50 different search engines. The search data shown is a record of the number of times the exact keyword or phrase has been searched over the last 12 months across all 50+ search engines. You would assume that the search volumes would be much larger than any other tool because of Keyword Discovery’s access to data from over 50 search engines, but the fact that the keyword matching option is set to exact, greatly reduces the total volume. Unlike Google’s tools, Keyword Discovery does not collect or show data relating to paid search, such as Suggested Bid, Cost per Click, and Competition. Data is collected daily, but updated monthly. Although you must pay the subscription to have access to all the features, Keyword Discovery does provide a free alternative.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.