sorry guys, i'm just the newbie to the iphone and I just got my phone jailbreaking so i'm wondering if all the application that you download from cydia on your phone is free ?
No, some are paid. If you try, you'll see. No risk, they won't send a bill or something.
Windows tool to edit .plist, .strings and .nib files: pledit.
Or use a website that will convert the binary form to a readable format.
On iPhone you can convert those files in a terminal with plutil (installed with Erica Utilities from Cydia), or edit them with iFile (from Cydia).