Windows isn't just for hard-core geeks or just for novices. Being the most popular operating system on the planet, Microsoft must design it to fit the widest possible range of users. Even its "techy" parts, like the Task Manager, sometimes need to be toned down for general consumption. But what if Windows were aimed only at the most technical of users? Process Explorer is the Task Manager that could have been.
Process Explorer can do all kinds of tricks that Task Manager just can't do. Case in point: Fire up Task Manager right now (hit Ctrl-Shift-Esc), and click "Show processes from all users." You will doubtlessly see a bunch of processes called "svchost.exe," all helpfully labelled "Host Process for Windows Services." Great, but what are those services? With Process Explorer, you can find out just by mousing over the process name.
This may sound like an obscure technical detail (and it is, a little bit), but it can also be very useful. For example, on my system there's a single svchost.exe process taking up over 250MB of RAM--a sizable chunk by any measure. Thanks to Process Explorer, I no longer have to wonder what all that memory is used for: I know exactly what services hide under that process, and can disable them if I want to. See all software downloads.
Another very neat Process Explorer capability: graphing GPU usage. Task Manager's Performance tab lets you see CPU and RAM usage only, but if you have a discrete graphics card, you have at least one more powerful (if dedicated) processor. Monitoring the GPU is useful not only for benchmarking games, but also for keeping an eye on applications such as Bitcoin or BOINC that use the GPU for general-purpose calculations. You can also see how much of the GPU's dedicated memory is used.
When going over the list of currently running processes, unfamiliar names will doubtlessly pop up. For example, do you know what csrss.exe is? I can guarantee this process is running on your system right now, under the nebulous description "Client Server Runtime Process." With Process Explorer, you just need to click the process and press Ctrl-M, and Process Explorer will instantly search Google for that process name. It's a very handy way to find out what's what in that long list.
Here's another classic Windows annoyance: You're trying to delete or overwrite a file, but Windows insists the file is in use. Process Explorer lets you see exactly what files are used by every process in your system, and also search for a file or DLL. In other words, you can track down the process that is using that file and kill it if you want to.