Yes, this is blatantly illegal, proceed at your own risk.
The App Store is missing one feature which I deem key, and that is the ability to try applications before you buy. While some apps offer “Lite” versions which accomplish the same thing, I’d like something a little more consistent.
Enter the jailbroken iPhone solution.
Requirements: A jailbroken iPhone / iPod touch running 2.0.2 or 2.1
Stage 1:
Open Cydia.
Install Open SSH.
Restart.
Stage 2:
SSH into your device. Default username is root, default password is alpine
Then navigate to the following directory: /System/Library/PrivateFrameworks/MobileInstallation.framework
Make a backup of the MobileInstallation file from the directory above.
Download the patched version of the file here for 2.0.2 or here for 2.1 (the 2.0.2 file might come up as a .dylib, if it does make sure and remove the .dylib extension after you copy it to your device)
Copy the patched MobileInstallation file into the /System/Library/PrivateFrameworks/MobileInstallation.framework
Set the permissions for MobileInstallation to 775
Navigate to /private/var/mobile/
Set permissions for the Applications directory to 777. Make sure reclusive is checked.
Navigate into the /private/var/mobile/applications/ directory.
Create a new folder named Documents in the /private/var/mobile/applications/ directory.
Set permissions to 777.
Navigate to the root level.
Open the Applications folder.
Create a Documents folder here as well. Set permission to 777.
Restart the iPhone/iPod.
Download an application from the app store (one of the free ones will work just fine)
Now you’re free to download and install cracked IPA’s! Obviously for legal reasons I can’t link straight to a source for cracked IPA files, but they’re pretty easy to find, just do some searching.