From ModMyiFone - Wiki
Apptapp is a super easy app which runs on your computer and lets you install Installer.app to your iPhone without any "hacking." Installer.app lets you install iPhone apps directly to your iPhone via WiFi or EDGE.
First off, make sure you have the following things:
--An iPhone.
--The AppTapp program. NOTE: The version I linked to here is for iTunes 7.4 and above, and an iPhone running the 1.0.2 firmware. If you have an older version of iTunes or the iPhone firmware, you should update. This does NOT work with 1.1.1 firmware version yet.
Step 1
Make sure your iPhone is plugged into the computer. Download AppTapp Installer and double-click it.
Step 2
The installer will ask you to make sure iTunes is off. Do so now, and hit continue.
Step 3
Choose your firmware version. Again, I recommend having the most up-to-date firmware, so 1.0.2. If you are using a different version, choose that.
Step 4
AppTapp will go through some steps of downloading, and your iPhone will display a few different screens, not to worry...
Step 6
That's it! You're done and ready to go!
Step 7
You will now have a new icon on your iPhone, the Installer.app icon! Open it up and install away. :D
If you haven't set up SFTP for drag and drop file transfer to your iPhone, I recommend you do so now - you'll need that for a few apps, like putting on ROMs for the NES emulator, or adding eBooks for the eBooks reader, etc.
