Mac: There are lots of system monitors out there. iStat Mini places all those stats right in your Notification Center so they're out of the way but still accessible.
With iStat Mini, you can watch your CPU usage, memory pressure, disk usage and network activity from your Notification Center. It's handy when you don't want another app clogging up your desktop or menu bar since it's easy to just load up Notification Center, take a glance at your stats, then close it again. It doesn't give you a lot of readings like the full version, but it displays the essentials that most of us need.
iState Mini ($2.49) [Mac App Store]