Windows only 
It’s normally safe to say that diehard geeks like to know what’s going on inside their machine. If you fall into that category then you’ve probably already tried tools that manage and monitor your PC, but there is also another free task manager that does a great job of adding functionality onto the semi-decent tool that’s included with Windows.
Extended Task manager is completely free, and once you get it running you’ll quickly notice that it looks a lot like the standard Windows Task Manager. The only difference is that this one is way more powerful. You can do things like monitor ports/IP’s that are going in and out of your computer, find out which app has a lock on a particular file, and more.
- Displays disk input/output activity with information about related applications
- Displays network port activity with IP addresses of external computers
- Displays information about applications that utilized most of the CPU or memory at any time presented on the charts
- Displays all files locked by a select process.
- Finds all processes locking a specified file.
- Includes “Summary” tab for quickly assessing the overall state of the Windows system
- Allows freezing individual Windows processes
- Offer various visual and functional improvements over the standard Windows Task Manager
How does this compare to Sysinternals’ Process Explorer?
In vista or newer the Performance Monitor will give you most of this information.
perfmon or perfmon.msc
“Allows freezing individual Windows processes”
…windows task manager has had that since at least as far back as Win2k.
Only interesting feature I see is “Displays information about applications that utilized most of the CPU or memory at any time presented on the charts”. But not quite interesting or useful enough for my purposes to install this.
I would say that it’s pretty close, except the one thing that I find really nice is that this will also show you which applications have a lock on a specific file. Previously I’ve used Unlocker to perform this task, but it’s nice that it’s included with this program.
Very true, but as I mentioned in the previous response the Performance Monitor will not let you kill an application that has a lock on a file. So this does have a little bit of an edge.