try Google's Keyword Tool free
Benefits:

• shows 12 months historical search trends
• can analyze a page or site for automatically recommending keywords
• they have the largest search userbase, and thus can offer great keyword depth
• gives estimated cost, competition level, and suggests potential negative keywords
• easy to export your list as an XLS spreadsheet

Negatives:

• this tool is well integrated into the AdWords system, so if your competitors have half a clue they should be bidding on these terms
• search engines may desire to show you terms which offer the highest prices instead of the highest search volumes
• the estimates are exceptionally rough

Google Suggest:
try Google Suggest free or try our Google Suggest scraper free
Benefits:

• Free, quick, & easy to use.
• Google has a huge database of search activity to grab this data from.
• Google's Toolbar auto updates and they added Google Suggest as a function directly to the toolbar in Google Toolbar 4 beta. They also have created a FireFox Google Suggest extension.
• Google updating their toolbar and potentially their main site to add this feature could cause the terms listed to increase in search volume