I was recently asked:
What do you recommend for tools other than Google, Yahoo and WordTracker to help with keywords finding out about popularity, seasonality and costs?
I replied: I think you have listed all the tools I use for determining cost:
There are other smaller keyword-based advertising tools but they tend to be specialized and target specific industries.
For determing the popularity of a keyword I use:
WordTracker and Google are by far my favorite, Both provide estimated search volume. Occasionaly I look at the clustering search engines to see if a keyword is related to a larger or more popular node.
Seasonality data is tough to find. Net traffic is growing so fast it clouds seasonal fluctuation and few sites keep data going back far enough to calculate a meaningful seasonality factor. The only tool I know of that has this sort of longitudinal data is Google Trends. And even then only very popular and clearly seasonal keywords seem to register a seasonality, e.g., valentines and Christmas trees.
There are certainly many more SEO and keyword tools available but, I think you can base the majority of your SEO decisions on data from these sources without compromising your chances of success.