ActiveLock from Alie Tan is a freeware Symbian program to protect the access to your phone applications using a PIN number. Use the default pin (12345) to configure the program, stablish a new password and select the apps you would like to protect.
The beta version is available to S60 2nd ed. phones (from FP2 and up) and 3rd ed. phones at this page.
best app i have