One of my readers suggested that one of the "must have" applications here should be AutoLock. AutoLock is a tiny free application that will automatically lock the keyboard after a given amount of time. Unfortunately AutoLock doesn't include any advanced features like automatically locking your phone when the slider is closed, etc. If you want more advanced features you have to pay for a more advanced application. AutoLock is available both for 3rd edition phones and older symbian phones.

To download the latest version of the software, visit this site