I use googles adwords keyword tool, but their are also paid alternatives such as wordtracker (which i have never used)

[adwords.google.com...]

They just updated it to actually show search volumes, but I don't think it's displaying all over the world yet, and also, even here in the US, sometimes I get the old one (with no numbers) and sometimes I get the new one (with numbers and a captcha)

It's probably good to discern trends, but don't swear by the numbers. In my long experience with AdWords, they aren't all that accurate.