WhoCrashed reveals the drivers responsible for crashing your computer. Whenever a computer suddenly reboots without displaying any notice or blue screen of death, the first thing that is often though about is a hardware failure.
PerfView is a free performance-analysis tool that helps isolate CPU and memory-related performance issues. It is a Windows tool, but it also has some support for analyzing data collected on Linux machines.
Lets you keep your eye on system resource usages of your PC. It supports 43 kinds of information including CPU, Memory, Network, and detailed HDD usages, so you may discover what's limiting your system performance in each occasion.