1. Wordtracker – Paid – Wordtracker periodically compiles a database of over 330 million search terms. Updated on a weekly basis. All search terms are collected from metacrawlers Dogpile and Metacrawler. Free Option – available here.
2. Keyword Discovery – Paid – Keyword Discovery currently collects search term data from over 180 search engines. The database contains approximately 32 billion searches from the last 12 months. Free Option available here
3. Google Adwords External – Free – generates keywords from Google’s database.
4. Keyword Country – Paid – Adsense keyword tool. There doesn’t appear to be a free option.
5. NicheBot – Paid – Searches across Keyword Discovery, WordTracker and Google. There doesn’t appear to be a free option.
6. Hitwise – Paid – Combination keyword research and competitive intelligence tool. Rather pricey, doesn’t appear to be a free option.