Here we'll try to address this issue by exploring some of the best (and free) application that can smartly lock, password-protect certain applications installed on your iPhone, Android and Nokia s60 Symbian device. Everytime a user tries to launch certain programs installed on your phone (e.g. Mobile Browser, Image Viewer, Inbox) he will be prompted to enter a password.
ActiveLock (Symbian) - Restrict Unauthorized Access To Apps, Files/FoldersActiveLock is a lightweight Symbian application that allows you to lock selected applications installed on your phone using a password. The program comes with neat UI and is very easy to use. Just get your free (unsigned) copy of ActiveLock for s60v3 here [24kB], sign it, and install. If you have problem downloading it, just Right-Click > Save Link As.
The nice thing about ActiveLock is that it works even if your phone has been restarted. So there's no way for your nasty friends to open the protected app even if they remove your battery.