Williamo’s Blog.

As the article says, this allows you to install unsigned applications without a hassle (S60v3) and apply patches:

Steps To Hack Nokia S60 Phone

• Install freeware X-plore, a file manager from Lonely Cat Games. Run X-Plore, press 0 and check first 4 boxes. Don’t exit.
• Install and run HelloCarbide by FCA00000.
• Switch to X-plore and check if you can see the contents of C:/sys/. If you can, then you are halfway done. If not, try step 2 again.
• Download CapsOnOff.rar by FCA00000.
• Copy CProfDriver_SISX.ldd from RAR downloaded above into C:\sys\bin of your phone. You can do this with X-Plore.
• Install CapsOn.sisx and CapsOff.sisx. Run CapsOff application to turn on the hack.
• Download SIS installer mod for Symbian 9.2 and copy installserver.exe file to c:\sys\bin\ so you can install unsigned applications
• Install ROMPatcher by Zorn.
• Whenever you need to install an unsigned application or patch your phone, just run the CapsOff application first. CapsOn brings it back to normal state.

(via How To: Hack S60 Nokia Phones)

However, if you’d rather keep your phone un-hacked and sign your own applications, you can get a developer account at SymbianSigned and use a tool like Barak’s SignMe! to sign unsigned applications for yourself.