One of the more popular monitoring tools, thanks in part to the amount of things beyond just hardware it can monitor, in the comments was Rainmeter. Lee writes:

Shinigamibob uses a wider range of tools to get a more in-depth look at difference aspects of his computer:

That being said, I do have Rainmeter so I can quickly see how much RAM or CPU is being used. For anything more detailed, I just go into the task manager and sort by RAM or CPU.

Not everyone relies on a plethora of third-party tools however, Frank gets by just fine (as do many readers) with basic in-OS monitor:

When I’m load testing or stability testing an overclock, its always the same couple tools. RealTemp and CPU-z. CPU-z has a lot more granular information than OpenHardwareMonitor does, but data that is only useful in a specific scenario. RealTemp is also very important in that it allows for the logging of temperature data. So even if I leave a load test running overnight and it happens to BSOD or reboot, I’ll always have the temperatures of each individual core.

When I’m gaming (the little that I do nowadays), its MSI Afterburner for GPU monitoring. The on screen overlay is incredibly useful in that the load, temperature, voltages, and fan speeds are shown directly on top of whatever game I’m playing. On top of that, it supports multiple cards just as well and shows if SLI or Corssfire is working properly (by checking the GPU usage and framerate). Its a small overlay on the corner of the screen so its never intrusive. More useful than Fraps for sure – the screen recording built has better utilization of CPU’s for better compression of video and wont eat up an entire disk for a 30 minute recording session (or cost you $35)

Most of those tools even have remote monitoring capabilities, but I prefer to use Mobile PC Monitor for that.

For more monitoring tips, tricks, and app suggestions, hit up the full comment thread.

For space, I use TreeSize Free to find the biggest files I can delete to free up space, I especially like the Portable version on my USB thumbdrives – otherwise My Computer shows drive size and free space no problems.