When I saw Matthew's article about Microsoft's new Process Monitor, I thought I'd give it a go.
Now there's a useful bit of kit. Okay, with the standard Windows task manager you can see what's running and what's hogging all your CPU. But with this little widget you can see what files are being accessed, what registry entries, and so on and so on. Okay, it's nothing you couldn't do before using tools like SysMon and RegMon, but having it all in one free, simple app is a pretty handy thing.
One more for my standard armoury of developer/sysadmin tools, then.