Negatives:

• Since Google is the largest engine many people may use this tool and bid up these terms.
• Like all things Google, they do not share specifics on how they gather this data or what it means to you.

ABC Tires:
For my ABC Tires example Google Keyword Sandbox recommended 4 different terms, and all of them have converted for me. It also listed a wide variety of semantically related search terms and modifiers, which can be used to help extend out a keyword list.
Google also has a highly useful keyword research tool within their AdWords interface when you log in. It automates keyword research based on entering a URL or site. It also allows you to find related keywords based on words you enter or words that are already in your account. The biggest downsides to the Google tools are that their search volume estimates suck and you can be fairly certain that some of your competitors will also be using the tools built into the AdWords system. Based on using all of these tools I believe that combining that tool with the SEO Book keyword research tool is just about all you need to use.
Overture (which is being rebranded as Yahoo! Search Marketing) Keyword Suggestion Tool:
try the Overture Keyword Suggestion Tool free
Benefits:

• It is quick, free, and easy to use.
• Overture has a large partner network to gather data from.
• Overture also offers a view bid price tool
• Overture offers search research by geographic region

Negatives:

• Many bid management tools and rank checkers scour the Yahoo! network, causing traffic numbers to be heavily skewed toward high value search terms.
• Overture does not seperate out plural and singular versions of keywords.
• Does not show related terms, only terms containing the exact phrase you search for.
• This was the original free search term suggestion tool, so everyone and their dog who does keyword research looks at this data.

ABC Tires:
For my ABC Tires example Overture only showed the four most common terms, all of which lead to many conversions, but were some of the most expensive terms.
SEO Book Keyword Research Tool:
try the SEO Book Keyword Research Tool for free
Benefits:

• Free.
• Easy to switch between markets.
• The results cross references many keyword research tools (almost every tool listed on this page - and a few that are not) so you can mix and match using them quickly and easily.
• Also links to different information systems (news search, directories, blog search, tagging sites, etc.) to help you find useful resources and things people are discussing in your marketplace.

Negatives:

• Driven off of Overture, so any problems with Overture's data occur in my tool as well.

WordTracker:
subscribe to WordTracker
Benefits:

• Helps you find or think of related terms by offering a built in thesaurus and lateral search. The lateral search looks at page details such as keywords of pages which are thought to be competing or related to your topic.
• WordTracker allows API keyword research access.
• Has more features than most other keyword research tools.
• The added features, such as showing the lateral terms, saves me a ton of time by showing me many related terms and useful modifiers I could or should be using.
• In the grand scheme of things the cost is not that expensive. It also has flexible pricing which allows you to subscribe for about $8 for a day or $250 for a year.
• Since it grabs it's data from Dogpile and MetaCrawler (a couple meta search engines) it's data is usually more clean than data from systems which collect data from networks that rank checkers and bid management tools work on.
• WordTracker offers a free trial, which the free Digital Point Keyword Suggestion tool queries.

Negatives:

• It is easy for competitors to spam the WordTracker database.
• Only grabs data from a couple meta search engines.
• Last time I checked it they had 305,005,525 queries in their database from 95 days, which is probably less queries than Google serves in a day.
• I do not think WordTracker sorts information into regional databases.
• Since they only gather data from a small sector of the web their data sampeling errors are magnified. Sometimes they will make low volume terms seem more important than they are, and other times some terms will not show up.