If you would like to mod your apps on your iPhone or iPod touch here's what you'll need-

1. A Jailbroken device.
2. Cydia
3. An app of which you would like to mod (I am using Zombie Farm)

Step 1. Go on Cydia, on the bottom go to 'Manage', then hit 'Sources'. Now you need to press edit, then add (both at the top) at this point it should let you enter a URL, enter this- [ Register or Signin to view external links. ] it will go through an installation, once installed open the source, (it will be under sources in Cydia) then you want to scroll down until you reach 'iFile', tap this and hit install in the top right corner. You have now completed step 1!

Step 2. Open up iFile (it's now on one of your pages of apps), All of the apps are represented by numbers, your gonna have to find the app you would like to mod, go through each one, each one will have the name of the app with .app ex: ZombieFarm.app
Ok you need to open that file.

Step 3. Ok everything from this point on only applies to you if you are modding Zombie Farm. So, first your gonna wanna make market items free or cost little to nothing. to do this you need to scroll down the list until you find 'Market.plist', tap this.
Ok so to make it simple, you wanna find whatever item you want to reduce the cost of, do this by tapping the magnifying glass at the bottom left corner. you can then search for the item. Once you find it, hit edit at the top left corner. Then look for 'cost', (it should be a couple lines down from the item name), under cost it should say <integer>a number</integer>, change the number, if the item you want normally cost brains, find where it says 'brains needed' for the item, under which it should say <true>, change this to <false> you can do the same for growth time of zombies. the number under growth time is the amount of SECONDS it takes to grow, i would not set this to under 60, because if so your zombies will wilt VERY quickly. You can also edit items purchased with the In App Purchase system, under the item find 'In App Purchase' under it should say <true>, change it to <false>

Step 4. In this step i am going to show you how to mod the XP needed to rank up. Ok, so first you need to find 'Playerlevels.plist', open this, hit edit, then between each <integer>...</integer> you can put whatever you want, i set mine starting of at one and then increasing by one as i went up.

Play around with the code a little bit, there is other stuff you can mod, i'm just tired so i don't feel like typing anymore, i might update tomorrow. Currently I'm trying to mod the invasion time but i can't seem to find where it specifies in the code. Ok i guess i'll put a list of stuff you can mod and see if you can figure it out.

Text on invasions (what they say when you invade)
Population of invasion area (how many people you must kill)
Farm area
Add your own levels

Any mods wanna sticky this?

*4/20/10 Update*
I have gone through about ten different apps and attempted to mod them. I found that you can mod Plants vs Zombies. On this app you can mod the characters text (ex: Crazy Dave). I am still searching for other apps that can be well modded for an advantage rather that just to create funny text. Oh and i just realized that in step 3 when you change <true> to <false> I left out the slash, the slash belongs right after the word true and after the word false, between the >. I tried to fix it but every time i submit the edit it goes back to the previous state.