Mac OS X only: Free application X Resource Graph (XRG) adds a customisable system monitoring dashboard to your Mac desktop. XRG shows real-time graphs of everything from your CPU and memory usage to system temperature and stocks (like the financial kind). It's a very attractive piece of software for what it does, and better yet, the look and feel is completely customisable. If you're a fan of system monitoring apps, XRG is worth the download. On the other hand, if it's not quite your cup of tea, check out previously mentioned iStat Menus. Windows users, try Moo0 SystemMonitor. Thanks PeterPumpkinEater!