1. Search engine keyphrase analysis tools

• Google Keyword tool (free access) - great for finding relative importance of search terms related to a keyword including synonyms. NB. Don't miss the "Related Sites" tool on separate tab shows keywords related to your site. Check you have selected the right term.
• Google Traffic Estimator (free) - Tool for summarising volume and CPC for up to 5000 key phrases - can be filtered by country. See my Google keyword tutorial for a guide on the Google Keyword tool and Traffic Estimator.
• Google trends tool - great for identifying peaks, periodicity or long-term trends in how your users search for you or your products in different countries. See my tutorial How to use Google Trends
• Google Suggest - A new tool in Google labs that gives keyphrase suggestions according to the number of pages on that topic.
• Yahoo! UK Search Marketing (formerly Overture) search term analysis tool - NB. Tool not currently supported or recommended. Formerly, the original and easiest to use keyword tool useful for finding numbers searching on key phrases and related keywords, but remember it excludes Google and Microsoft Search.
• Nichebot - Paid tool but US based - integrates Google and Wordtracker and shows competitiveness for limited number of related keywords. The next two are better known and have UK databases.
• WordTracker Gives similar results to Overture, but accross all networks. US-based, but with UK module. Based on usage of metacrawler meta search engine which is not a representative audience for mainstream search engines.
• Keyword Discovery (paid) Paid tool similar to Word Tracker from Trellian but offers sophisticated trend tracking and regional - now with UK module. Again issues with sources of data - from Metacrawlers.