Word tracker does not use Yahoo or Google data. WT is good for finding words. You should not use the data from OV or WT as an absolute. It is best just to compare the OV numbers with other OV numbers and the same with WT. It is very useful if you know how to look at the data. Also you have to be careful with OV they include plurals and singulars together with broad match. I use both all the time.
as ogletree pointed out, the value of the numbers WT/OV provides are relative. You can find the most popular words with a given root. Only you, by ranking 1-10, can know just how much traffic they push. Eventually you'll realize there isn't really any value to knowing how much traffic terms push overall, just whether or not you're getting a slice. You'll quickly gain a sense of produced traffic per keywords in relation to the count numbers WT gives, once you have rankings. WT just gives you a hint at which rankings you might want to go for.
I find WT better for finding 'real world' search queries. It gathers its data from a couple of meta-engines, like dogpile and mamma (don't quote me on those being the actual sources), and they don't affect the data in any way, unlike OV. afaik, OV concatenates plurals and 'match-driver' equivalents into a single term. WT even lists capitalization differences.
anyway, given all that, WT's site has been weird recently, and today I cannot get to it at all. :(
Another great way to get keywords is to use overture and/or adwords. You don't have to use it all the time just enough to get some info. Bid up at night or just for one week or one day. Turn on the broad match and look at your logs.