When marketers start tinkering in search engine optimization (SEO) or in paid per click (PPC) advertising the lure of "free" draws in new search marketers like moths to a flame. The most well known "free" tool out there is the Yahoo/Overture Inventory tool. Aaron Wall of SEOBook.com has also has an expanded version of the Yahoo! tool, by adding in the subjective search totals from Google and MSN via a calculation of roughly twice the number of searches in Yahoo! for the Google total, and 1/2 the number of Yahoo! searches for the MSN total.
These tools are great to give a general idea for keyword research and at first, seem like the best source of information out there. They are also a great way to find alternative keywords that you might not have thought about. If you are a search marketer who is just starting out, these tools are a great help in helping your clients, whether they are internal or external, figure out just where it is they want to go keyword wise.
At some point, search marketers have to move beyond the lure of the "free". Why? Skewing of the keyword is why.
The best example of skewing with the free keyword tool from Yahoo! is the term "Halloween Costume". If you started to build a site for your Halloween Costume store back in February and started planning for your PPC budget in February and you were utilizing the Yahoo!/Overture tool, you could be highly disappointed come August, September and October. You will likely have blown through your budget if you based it upon the search numbers from February and March since no one is "searching" for Halloween Costumes at that time of year.
This is the primary reason for subscribing or purchasing a keyword tool that can give you a full picture of the traffic keyword produces in searches. I've worked with a few tools, and no one tool is going to give you exactly the data from all the search engines, however, the tool that comes closet is Trellian's Keyword Discovery tool.
Keyword Discovery is by far the best tool out there to give you a year's worth of information on a keyword. You can see the trending, the peaks and valleys, and also which engine the keyword is strongest in. KWD also allows you to see views of keyword performance in EBay and shopping engines like Bizrate, Froogle, Nextag and more.
By investing in keyword tools, you can punch up your value to your clients as you can understand a keyword's life cycle. You can also understand they nature of the keyword's performance in the various engines and shopping sites with tools like Keyword Discovery. Utilizing these two powerful tools can put your far ahead of other search marketers, because in the same amount of time they are utilizing the free tool and calculating "projections", tools like Keyword Discovery have given you at least 3 times as much information. In today's world it is all about working smarter, and smart is investing in the tools to get the job done right.