1) Using your Computer
Okay, now if you want to get free apps, you can just download a cracked version on the web. I'm not giving exact websites, but you can perform a Google search for "the name of the app ipa" and find a website for the app you want. Once you've found the app, you can't just drag it in iTunes and sync it over, you have to use an alternative method.
1) Download iFunbox from the website
2) Connect your device to your computer and open iFunbox
3) Your device should be on the left pane. Click on "User Applications"
4) Click Install App at the top
5) Locate the ipa file you downloaded and click OK
6) iFunbox will install the app
2) Using your device itself
This way is the easier method of getting free apps. Just download one of the following Installous alternatives in order to do so. Remember this will only work if your device is jailbroken.
App Cake - Requires iOS 4+
1) Open Cydia. Go to Manage-Sources-Add.
2) Add source: http://cydia.iphonecake.com
3) Search for "App Cake"
4) Download App Cake/App Cake +
5) Reboot device and App Cake should be on your springboard.
App VV/vShare - This is a app made my Chinese developers, but you can change the language to English. It has a pretty good interface and the downloads are direct links from them, not like the ones that you have to wait like 20 seconds and type in a captcha to download.
1) Open Cydia. Go to Manage-Sources-Add
2) Add source: http://repocydia.com
3) Search for "App VV or vShare (they are the same)
4) Download App VV/vShare
5) Reboot device and App VV/vShare should be on your springboard.