Here are 3 links to 3 YouTube videos, all require that you download cheat engine 5.5, there are no "typical" game cheats available for this game, it does however have several unlock-able phrases that change what the Zombies are wearing and what they do upon defeat listed at the bottom of this answer.
1) Unlimited sun
http://www.youtube.com/watch?v=o59FFeE7Z2Q
2) Unlimited coins
http://www.youtube.com/watch?v=mTq6rO63I2Q&feature=channel
3) Unlimited height on tree of wisdom
http://www.youtube.com/watch?v=bnEgUMGG9-Q&feature=channel
Future: add futuristic glasses to all zombies
Dance: makes the zombies dance
Daisies: grows daisies where zombies die
Mustache: gives all zombies mustaches
Pinata: candy pops when a zombie's head falls off
Trickedout: makes the lawn mower look cool
Sukhbir: Changes the zombies' call for brains
Plants vs. Zombies 2 available now for free
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Plants vs. Zombies was first released for PC and Mac on May 5, 2009
yes! you can check it on plants vs zombies website
Use cheat engine.
you can't add cheats on plant vs zombies but you can type on the screen the normal cheats like daisies,future etc.
cheat engine
pc future pinata mustache dance some only work after widom tree is 100 feet taller or more.
Plants vs Zombies for Xbox360, PC, and iPOD
plants vs zombies
Plants vs. Zombies 2: It's About Time has already been released on iOS but PC version's date of release is still unknown. There will also be another Plants vs. Zombies game with subtitle: Garden Warfare. It will be released on PC, X360, XONE and PS3 in 2014.
There is a list of the various codes you can use on Chewy Central.