I'm looking for a tool that can show the number of monthly searches on a particular keyword over the last six months or so.
Does this tool exist?
If not why not?

www.wordtracker.com or www.keyworddiscovery.com is the closest you'll get

or you can run an adwords campaign to get your own real data

I've tryed the wordtracker trail and adwords which tell you how many searches now, but not six mothes ago. I have used the overture keyword tool as well but they show data from about a month ago only. If a person could take a keyword and find out how many searches going back a year or two, that would be much more useful.

Well ... for the ones you're interested in, you can start to track that now, and build a historical record going forward. I'm sure bits and pieces of what you're talking about exist, from people doing just that, but I don't know if there's a comprehensive data warehouse.

Have you played with the new Google Analytics interface? The keyword tool in there is really handy. This only tells you about what you've been found with in the past, but combined with their Webmaster Tools ( which shows what you come up for, and which of those people click ) you can get some good ideas.

I know that's not what you're looking for, but I don't know where to find that. And this has actually turned up some good info for me. A few searches I turn up on the top of the second page, for example. I don't know how often people search on this stuff, but I do know I can get a part of it without too much effort.

yah, run one month of adwords you will get acurate information. Just set your daily limit low.

The major problem with tracking keyword searches over 6 months is that the database size for such a task would be far too large to be effective.

The minor problem is that 6 months really doesn't take spikes or traffic surges into full account based on seasonal issues.

ForrestCroce is right...the best source of keywords and ideas tends to be referring logs from search engines. You'll often find longtail phrases and terms you can use to generate shorter, higher-traffic phrases and terms.

This is actually one of the reasons behind the Search Query Info section of my site...I take the 5 goofiest things people have found my site under in a 7-day span every Sunday (actually, it works out to 6 because I prewrite the posts on a Saturday) and attempt to find useful content or answers for those things.

By doing so, I'm not gaming the engines because I'm already found under these terms; I'm catering to the users because I'm giving them information I didn't previously have. But the side effect is that I often pick up additional goofy keywords and phrases to work from. And so the cycle repeats, and so the traffic builds.