Here's a list of some free pool drills I've found on the internet.The first link leads to a Snooker website which has drills to check for a straight stroke.The drills on this site are also good for pool.Once your stroke is straight proceed to the other links for some free drills and Diamond Systems.
Here's the links:
(press read more for diagrams)
(go to instruction section)
DIAMOND SYSTEM LINKS
If anyone knows of any other good sites for free pool instruction, feel free to add to the list RJ