Yes, "Plants vs. Zombies" is free to play on several flash game websites. All you need to do is look up "Plants vs. Zombies" online, and you're sure to find a free-to-play version on a flash game website.
"Plants vs. Zombies" is a classic Tower Defense game that can be played on a multitude of websites. It became such a popular rendition of the classic genre that the creators decided to make for-pay versions available on several consoles and PC. If you like the flash game, check out the console or PC versions for added features and new gameplay experiences.