First of all you’re going to need a Jailbroken iPhone with Cydia installed to be able to install any iPhone App for free.
If your iPhone is not yet jailbroken then check out this link for an easy tutorial. If you have AT&T then go for 2.2.1. If you use another provider and unlock your phone go for 2.2.
Jailbreak Tutorial: http://www.iclarified.com/entry/index.php?enid=1881
1. Once jailbroken, open Cydia, go to Manage, Sources, and then add this source: http://cydia.hackulo.us/
2. In Cydia, search in the search section for miPatch. (To unlock your iPhone 3G, add this source: http://apt9.yellowsn0w.com/, go to search, and search for yellowsn0w, download, reboot, and your iPhone is unlocked![Only works for 2.2 iPhone 3G, if you are already on 2.2.1 don’t restore, because unlocking it won’t work)
3. Once miPatch is installed, go to youtube.com/how2doitall on your PC. He has a bunch of videos with cracked iPhone app links.
4. Search for the free iPhone apps you want.
5. Download it and save on to your desktop.
6. Once the download is done, double click it.
7. It should go in to iTunes, and then plug in your iPhone, and sync it.
8. Congratulations, you just got your free iPhone app.