iStat Menus is a system monitor for Mac that adds graphs and stats to your menu bar. It details system stats like CPU, Disk and Memory usage as well as system temperature and many more useful stats. There’s a 14 day trial, after which you’ll have to pay $16. If you don’t need to keep a constant vigil over your Mac’s vital signs, there’s also a free dashboard widget, iStat Pro that provides just about all of the same stats. For those who would like to monitor their Mac from their iPhone, iStat for the iPhone connects to macs running iStat Server (a separate and free application) and allows you to keep an eye on your systems health remotely from your iPhone. The app also allows you to see your iPhone stats including; memory, disk space, WiFi and Cell IP addresses, uptime & load averages.