Google is now providing trendinig data for every keyword in their database.
Try this:
1. Visit the keyword tool
2. Enter a keyword such as “PHP”
3. From the drop-down menu select
Voila!
Google will show a bar shart that shows the total search volume by month, as well as the months in which the search volume is the highest.
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<img alt="" style="border: 0;" border="0" src="http://www.pheedo.com/img.phdo?i=b76f1ac9d4c1ee9fd21c22e1109a1cf0"/>
More info