Wordtracker is another service that requires an annual subscription to utilize all of its features. Their database is made up of 330 million search terms, collected from the major metacrawlers, Dogpile and Metacrawler, and is updated weekly. The search data they show is a record of the number of times the exact keyword or phrase has been searched in the United States over the last 365 days. As in the case of Keyword Discovery, this difference in default keyword matching type causes Wordtracker to display search volumes significantly lower than what Google’s tools show.
If you don't know the difference between head terms and long-tail keywords, let me explain. Head terms are keywords phrases that are generally shorter and more generic -- they're typically just one to three words in length, depending on who you talk to. Long-tail keywords, on the other hand, are longer keyword phrases usually containing three or more words.

In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).