There are a number of keyphrase research tools available for anyone to use, these tools allow you to find out what phrases users are searching for on the major search engine Google, Yahoo and Bing . In addition to tools provided for free by the search engines there are also some pay off tools as well. Personally I use Google's Keyword Tool the most, I like its simplicity and in the UK Google has 80% of the search market so using its tool will give you a good idea of the volume of searches in the the UK.
Simple and easy to use tool, enter a seed word (for example 'websites') and it will display searches related to that keyword and the estimated number of searches. The new tool (on beta at the time of writing) gives estimated volumes for each month of the year.
Free Keyword Suggestion Tool from Wordtracker
Personnally I've nver got on well with Wordtracker's paid tool (see below), the free tool is esay to use and gives you simple volumes on the top 100 phrases returned from your search.
Keyword Forecast from Microsoft adCenter Labs
I really like this tool it gives useful information on past volumes, th only problem is that the figures are out of date (at the time of writing it was reporting figure from 2006 and 2007). It also reports information on age and gender which is very useful to know.
There are a number of pay for research tools that offer keyword research and in some cases competitive intelligence as well:
What to do with the information
The information these tools provide is invaluable, it tells you what people are searching for. The process I follow is to create a 'seed' list of words that I feed into a keyword research tool, I then save all the resulting phrases into a spreadsheet to create a keyphrase repository for each website. The Gogole tool will give you seasonal variation and competition (the number of websites competing for this phrase).
The key repository should then be broken down into different target groups, so for our website this might be content management, ecommerce, hosting etc. And within each group you identify target keywords, taking into account competition and relevance - the keywords you target must be relevent to the product or service you want to promote. I suggest dividing your list into primary, secondary and tertiary target groups.
You can also use keyword research to find new products and service related to your current offering that you may not have thought of. For example, if you are a CD retailer and find that there are thousands of searches for a CD you don't currently stock.
Another useful tools for looking at season trends in keyphrases it Google Trends. This tool shows you the season variations, top Regions, Citys anfd Languages for keyphrases. It also shows news articles on the volume graph to show links between search volumes and mentions in the media.