Rob Taylor runs an internet marketing video blog and has recently posted an interesting alternative to traditional keyword research tools.
He suggests using Amazon.com (not .co.uk) and their ’search inside’ feature. This, for any given book, provides you with a list of ‘Statistically Improbably Phrases‘ (SIP) - these are phrases which Amazon considers significant. It works by looking for phrase matches across all books (that have the ‘Search Inside’ feature) - and where a phrase appears in a large number of books, it is marked as a SIP.
Gathering keyword phrases like this and using them in optimised content could really pick up long-tail searches around particular topics.