Process Explorer is an awesome task manager replacement. With it you can view all applications running on your computer (even ones Windows TaskMan hides), get more information about your application (working memory, peak working memory, etc.) and more! You can even see which applications are sub-processes of a parent application. It's great to use on servers because you get much more control over processes and you can see more.
There is one major problem though. If you play games on your computer, and the game uses SecuRom disc protection, the game will not run when Process Explorer is running. You will have to disable Process Explorer from replacing the Task Manager and restart your computer. The reason this happens is because Process Explorer includes a Debugger and SecuRom detects it as a hack tool. Aside from that, its safe to use. The original programmer was hired by Microsoft a few years ago and the download site is on the Microsoft website.
Process Explorer is Freeware and works on Windows XP and higher and Windows 2003 Server and higher.