Eraser
http://heidi.ie/eraser/
A tool to erase files, directories, complete disks or empty disk space only.
It can use several patterns, some official, and you can define your own.
It's open source.
Trader's Little Helper
http://tlh.easytree.org/
THE tool if you use lossless audio a lot.
Encoding, decoding, verifying checksums, fixing sector boundary errors, etc.
Spybot Search & Destroy
http://www.safer-networking.org/index2.html This 1 shouldn't need an explanation.
SpywareBlaster
http://www.javacoolsoftware.com/spywareblaster.html A tool to prevent a lot of spyware and mallware from harming your system.
Unknown Devices -
http://www.halfdone.com/Development/UnknownDevices Identify unknown devices in your pc