For many years, the Overture Keyword research tool was the main source of data people had about search terms. It ostensibly showed what key phrases had significant volume on Yahoo! Search.
Now Yahoo has gone, and canceled the free access to the Overture tool, which many SEO keyword tools (like Aaron's) had relied on.
Here's a good substitute: Wordze. I've used wordze for 3 months. It's not free, but it is very useful for doing keyword research.
My favorite features are:
For example, a dig query for "projector" took about 12mins to complete. It returned 4848 words, all very relevant phrases, mostly 2-5 words in length. It provided a frequency count, estimated volume, and a KEI for the top 200 or so. Here's the top 10:
|3m overhead projector||119||2786||55.37|
A wordrank query for "humidifier" returned a lists of domains, counts of links, types of links (Edu links & Gov links), domain and URL pageranks, along with search demographics by country, and city
Wordze also has filters and options for finding keywords in profitable niches such as Adult, Drugs, Gambling, Warez & Hacks. This means you can get lists of those types of keywords if you are into those areas.
Most of the Wordze tools work on a batch basis - you put a query in, and it works on it in the background, so you can quickly enter 10-20 queries, and wait about 20mins for them all to be completed. Then you go download the results from the download manager.
Support on the tool is good. I've gotten several replies in under a few hours from Levi - the author, whenever I had a question.
The downsides to Wordze? Sometimes you have to use multiple steps to really expand a keyword list. For example, it might return basically the same set of keywords for related terms like "refigerator" and "frigidaire". Then you may need to use the refining tools to get a more expanded list for the terms. Doing this is a pretty good way to find long-tail keywords that competitors may not be using, however.
Wordze doesn't really provide any bid / cost information about keywords, but it does indicate competitiveness. I never found the Overture bid estimates to be accurate at all. The only way to tell how much stuff will cost is to go bid on it.
I use Wordze mainly for SEM - quickly building lists of keywords to buy. I think most people that use it probably do SEO research with it.
Wordze costs about $35/month and payment works with PayPal. To me, it's a no-brainer, because if you do any serious keyword research, it can easily pay for itself in 1 day.