Wordtracker
Wordtracker is a great addition to Google adwords keyword tool, it collects data from dogpile and metacrawler. Although its data is only a small fraction of the entire search query data pool, it can give you the exact search volume on each keyword, and also many relevant long tail keywords. I would say wordtracker is often more inspirational than Adwords keyword tool, because of these highly relevant long tail keywords.

Note: wordtracker’s free version only shows US search queries, if you need UK search queries you will have to pay.

It is good at understanding these factors:

•    Relevant keywords
•    Search volume