I’ve often been asked “What are your favourite tools for keyword research?”
I usually spend the time to come up with a measured response, covering all the possible free tools which people use, and why they’d want to perform competitor analysis; monitor historical search trends & suggest alternative keywords. Whilst this is all useful information it’s easy to loose focus on what is essentially the most important resource.
Google provides my preferred set of tools for performing keyword research! After all, you wouldn’t design a website without checking that it renders as intended in IE as for the majority of users that’s their browser of choice; so why wouldn’t you use the same search engine as your primary source of information. Google’s keyword tools query their own databases & as they dominate the search market; they tend to be more accurate than other free keyword research tools.
The discrepancies between Google Adwords Keyword Tool and Paid Search results are intriguing enough to make mention. It’s important to consider that Google leans toward  making their numbers appear larger than in reality as the main users of Google’s keyword tools are either current or potential Adwords customers. Although it isn’t free I have often found it useful to start an Adwords campaign for a client to get reliable conversion data for Keyword lists. This paid keyword data is invaluable for keyword choice for natural search optimisation.
Google also provides search parameters which are useful for Keyword Research & whilst you’ll normally be performing this by hand you can be confident in the integrity of the data. There are more parameters than what I’ve listed but I find these provide the most insight: allintitle: to search title tags; allintext: to search page text; allinurl: for searching URLs and allinanchor: to find sites linked to with specific anchor text.
Free keyword research tools:
http://freekeywords.wordtracker.com/ keyword suggestion tool generally considered to be accurate
http://www.keyworddiscovery.com/search.html keyword suggestion tool
https://adwords.google.com/select/KeywordToolExternal Googles Keyword Tool; aimed at Adwords users
http://tools.seobook.com/general/keyword-information/ gives a comparable amount of information as Googles Keyword tool
http://tools.seobook.com/keyword-tools/seobook/ very informative provides information from many different search engines
DISCLAIMER: The majority of my freelance work has been for small companies in niche markets with only a handful of clearly defined products and services; if you are dealing with large companies your requirements are likely to be different and I would recommend using some of the paid keyword research tools.