System Informer is a free, powerful, multi-purpose tool that helps you monitor system resources, debug software and detect malware.
Features:

1. A detailed overview of system activity with highlighting.
2. Graphs and statistics allow you quickly to track down resource hogs and runaway processes.
3. Can't edit or delete a file? Discover which processes are using that file.
4. See what programs have active network connections, and close them if necessary.
5. Get real-time information on disk access.
6. View detailed stack traces with kernel-mode, WOW64 and .NET support.
7. Go beyond services.msc: create, edit and control services.
8. Small, portable and no installation required.
9. 100% Free Software

Note: it appears to be the successor of Process Hacker.