Software to diagnose faulty computer hardware?

Etylowy writes: Over the years I have repaired my own, family and friends PCs many, many times. While in most cases it turned out to be restoring system after malware/user/windows made a mess or simple cases of "follow the smell of smoke and molten plastic", there were some nasty ones where computer mostly works. By mostly I mean: you can boot it up, it might even work for a while but will crash way too often to blame it all on Microsoft — what do you do then? Once you strip it off any extra hardware (which with todays motherboards with pretty much everything integrated might not be an option) you are left with CPU, motherboard, graphics card, RAM and HDD. You can test HDD, you can run memtest86+ to check RAM, but how do you go about testing CPU, motherboard and graphics card trio to find which is to blame? Replacing it one by one isn't really an option. Do you know any software that would help the way memtest helps with RAM?