Microsoft has recently released a plugin tool for Excel 2007 that might rival all available engine keyword tools. It is called AdIntelligence and it provides a lot of interesting data.
The two features I find the most promising are the monthly traffic tool, which takes keywords you input and returns historical search data from the Microsoft search engine Live.com. The tests I have done show huge variations in month to month traffic, which is very interesting, if the reported numbers are accurate.
Here’s an example of a few car insurance keywords with a year of historic data, as well as 3 months of predicted performance. (Click the image to see the full version.)

This level of transparency and the amount of data provided directly by a search engine blows apps like the old Overture keyword tool out of the water. The spikes in data are curious, though. Take for example “buy car insurance online” which vaulted from 9,096 queries in May to 105,523 in July. This would not be as suspicious to me if the same fluctuations were seen in other related keywords, but that’s not the case in this instance. It makes it hard to chalk such a fluctuation up to seasonal performance changes. Data integrity remains a question. Either way, this feature is very promising (the tool is still in beta) and should help anyone at least get an overall idea of the traffic behind their potential keywords.
The other interesting feature is a monetization breakdown, which takes a list of keywords you provide and returns the impressions, clicks, average position, CTR, and CPC for all advertisers on AdCenter. I was fairly surprised to see so much information freely provided by Microsoft, considering how closely guarded the engines traditionally keep this information.
Here are some of the same car insurance keywords from the last example:

According to this data, there were 971 clicks on the exact keyword “car insurance quotes” in the last month, at an average CPC of $15.00, all the advertisers bidding on this keyword averaged a 3.05% CTR.
Other features of AdIntelligence not covered here are a Demographic and Geographic breakdown of keywords, as well as several keyword research tools, that take either a URL or original keyword and return a list of similar keywords. There is also a tool that will return keywords that advertisers are bidding on related to your original keyword.
AdIntelligence is a plugin for Excel 2007, you can download the beta here. There is also a 60 day demo of Excel 2007 available at the link if you don’t have it. I highly recommend trying out the tool, and I hope Microsoft continues to add new features.