Secunia is a respected security service provider that tracks vulnerabilities in more than 20,000 applications and operating systems. To find information about the latest vulnerabilities, you could subscribe to Secunia's mailing lists, but if you want to know whether there are known security issues for the software installed in your computer, install Secunia Personal Security Inspector.
The Windows application scans your computer and it lists the insecure programs, information about vulnerabilities and links to the patches. In most cases, Secunia provides direct links to the latest updates, so they are easy to download and install. To find more information about security problems and to list all the software from your computer that needs to be updated, switch to the advanced interface.
Secunia collected data from 20,000 users of the software and found at least one vulnerability in 98.09% of the cases. "By insecure program it is understood, that there is a newer version of the program available from the vendor that corrects one or more vulnerabilities, but the user has yet to install the secure version. A vulnerability in a program can be exploited by hackers to anything from compromising a PC, to automatically install trojans/viruses, to sniff out private information (passwords, credit cards information, etc)."
Not all applications include auto-update and users have to manually update to the latest versions. Google is one of the companies that thinks it's important to update software without any user intervention, that's why most Google software has an auto-update feature or is integrated with Google Update.
Secunia's software focuses on updates that solve security problems. More comprehensive solutions for updating your software include UpdateStar, FileHippo Update Checker and Appget, but none of them is very reliable.