Update (July-8-2008): Google Keyword Tool Updated To Show Search Volumes
This question pops up every time there is a discussion about keyword research. As always the answer is…
None!

It is not very difficult to see why.
There are hundreds of search engines and people from all over the world are doing searches all the time. There is no way some one can actually collect this data, create a database of search phrases and tabulate the number of times each keyword phrase was used.
Even if someone were to attempt this, where would they get the search data from?
Search volumes from search engines
Google will know for certain how many searches occur on Google for each keyword phrase. This will be significant given that Google is the most popular search engine. Unfortunately they don’t actually publish the search volume data. Recently Google Trends have been updated to show some numbers (if you are signed into your Google account). However it still doesn’t show the actual search volumes.
The Google keyword tool shows some information on relative search volumes as a small horizontal bar. Again, no actual values are provided (Update 8-july-2008: Google has started showing search volume counts).
You can create a Google AdWords account and bid for terms. Then check the number of impressions for each keyword phrase. This will give you some idea about the search counts.
Another solution is to approach a smaller meta search engine and make a deal with them to obtain the search volume data. This can be extrapolated to estimate global search volumes. However you really can’t consider this kind of data as highly accurate.
Search volumes through ISPs
ISPs handle a large amount of Internet traffic and they do keep a log of all URLs that are accessed. So they are in a position to see what searches take place. However, this information is mostly localized. You will need to collect data from multiple ISPs from all over the state / country / world and tabulate it. This is not an easy task because ISPs will not release this information due to privacy laws. Again, some smaller ISPs may provide this information to private parties.
Stop worrying about the search volume counts!
Wondering about the accuracy of search volume counts serves no real purpose. Search volumes vary from day to day. Some keywords are highly seasonal and some phrases spike because of a current event.
Keyword counts are also polluted by Internet marketers doing searches as part of their research.
A good article on your site can attract a large number of visitors with each one using a completely different keyword search phrase. We have had hundreds of visitors a day coming to pages on our site with not a single keyword phrase used twice!
Instead of going after a few high search volume keyword phrases, find the top keywords and keyword phrases that are important to your field / industry. You can use the Google keyword Tool or a software like Keyword Strategy Studio (part of IM Toolpad) to do this. Once you have a list of the important keywords use them in your web page copy (as titles, headings and as part of the page text).
Spend more time writing useful articles and blog posts for your visitors and customers or improve your existing web pages instead of obsessing about the keyword search volumes. This will automatically create a large number of search word combinations on your site. The traffic will follow soon.