http://www.pcworld.com/printable/article/id,185679/printable.html
I've used four out of ten tools listed at the link above and highly recommend them to others...
Sysinternals Suite
Process Explorer and TcpView are my favorites
HWiNFO32
Never use it but it can be handy when you need to find out the hardware info of your computer
Crap Cleaner
Never use it but it sounds like it can do a decent job cleaning up a Windows machine
Recuva
Never use it either but it's good to know this kind of tool is available
FileZilla
Have used both the client and server version. The client not only works great for those who are not familiar with the command-line FTP but also when you need to transfer multiple files and multiple directories, it's simply a drag-and-drop
Virtual CloneDrive
Never use it but this could be a handy tool. It's good to know.
Defraggler
Normally use the one that comes with Windows but it does not hurt to know another one out there
ImgBurn
Never use it but it's good to know when I don't need my Nero anymore
7-Zip
Like it a lot. Alternative to WinZip and more
VirtualBox
Like it a loooooot. I've built multiple VMs with virtualbox for experimenting different OSes