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Number one is the Secunia Personal Software Inspector, quite possibly the most useful and important free application you can have running on your Windows machine.
It can be used to scan all the installed applications on the PC to determine which programs are missing security patches/updates.
The tool works by by examining files on your computer (primarily .exe, .dll, and .ocx files) for meta information on specific software builds installed. After examining all the files on the machine, the collected data is sent to Secunia’s servers and matched against the Secunia File Signatures engine determine the exact applications installed on your system.
It can be used to flag insecure/end-of-life software and find direct download links to missing security updates.
It monitors more than 4,200 desktop applications. More information here.