WakeUpOnStandBy

You can tell your Windows PC to automatically wake-up while you're not there and perform some functions, like opening a web page and running a file. The ability to set these scheduled tasks is built into Windows itself, but tools like WakeOnStandBy offer an easy way to set your PC's alarm clock. The software lets you set what time your computer wakes up, what it does afterwards (running a file, or batch file), and when it should go back to sleep.
ElectricSheep

We mentioned this completely mesmerising software in our feature The 101 Greatest Web Sites. The creator's website describes it as "a form of artificial life", that "recreates the biological phenomena of evolution and reproduction through mathematics". Did we mention it is also a screensaver? The app creates fascinating and constantly morphing abstract shapes and designs that evolve, depending on how popular they are with other Electric Sheep users
Threatfire

If you're not using an anti-virus program that protects against "zero day" threats, you're probably not getting very good protection. The big names in anti-virus are catching on with heuristics and reputation-based file scanning, but if you're using a free anti-virus program it's possible you're not getting the same level of protection. Threatfire aims to add extra protection by looking out for and stopping suspicious activity, rather than relying on virus signatures. It's no substitute for a solid security suite, but instead works alongside these programs.
Image Composite Editor

This app automatically stitches together lots of individual photos to create giant panoramas - something you can try with varying degrees of success in other apps, but Microsoft seems to have nailed with Image Composite Editor (ICE). The key is the software's ability to cleverly mask the appearance of any joining points between each photo - panoramas appear as though they were taken in one photo. The killer is the lack of any file limitations.
KeePass

So you store your password in a password protected zip file, or worse, an ordinary Word document? Not only are you risking having your bank accounts and PC logins exposed to spyware, but you're probably having to go looking for passwords whenever you need them. KeePass stores your logins in an encrypted database, and automatically enters your passwords whenever you need them. You can also run it from a thumb drive so you can take your passwords with you. Also available for Mac and Linux.
Got a favourite PC app? Add your suggestions in the comment box below.
Also see:
5 More Free PC Apps You Can't Do Without
5 Free Linux Apps You Can't Do Without