Erasing data but keeping applications

If you only used one or two user accounts, create a third admin one, login to that account, delete your user account(s) via System Preferences > Accounts > MINUS button when you select you account(s). Choose to delete the home folder and after that is done, go to Disk Utility, select the Macintosh HD volume (where your OS resides on) and click the ERASE tab and there click the ERASE FREE SPACE button and there select ZERO OUT DELETED FILES.

Also know, that keeping paid applications on there without giving your friend the installation media is considered illegal and thus it would be easier to wipe the Mac and show your friend how to torrent stuff.

You may also have to look for threads and guides describing unlinking your Mac from your iTunes/Mac App Store account (Apple ID).