Google Trends (free)

• Estimates relative search volumes for category keywords
• Demonstrates search trends over time
• Highlights key news events as they correspond to spikes in search traffic
• Enables you to filter by geo-location

Google Insights for Search (free)

• Provides detailed analysis on one keyword or a group of keywords
• Facilitates comparison by search terms, location, or time range
• Returns lists of top related searches and search terms that are rising in popularity
• Enables you to filter by geo-location and category (e.g., "health" versus "food and drink")

Google AdWords Keyword Tool (free)

• Enables you to enter one term to generate a list of hundreds of related queries
• Helps you quantify historical search volume, filtered by geo-location
• Displays search volume trends over time in graphical format
• Assists in building initial keyword lists for your AdWords campaigns

Microsoft adCenter Keyword Research Tools (free)

• Visually presents associations and sequencing among queries in the same session
• Detects and displays keywords that are similar or related to the search term entered
• Displays commonly misspelled terms or alternative spellings of the search term
• Forecasts search inventory and demographic makeup of term searchers

| More...