iStat has been one of my favorite widgets since the inception of Dashboard. Along with the ever-useful Easy Envelopes and WakeOnLan, it's one of the only widgets I use anymore. It's a great diagnostic tool when things aren't responding as fast as I'd like, or when something seems to be misbehaving. All the vital stats I need to figure out the problem are presented to me in an easy-to-read format.
As much as that's useful on my Mac, it would be nice to get that interface without invoking one of the more sluggish features of Mac OS X (Dashboard) or launching yet another application (hardly prudent when your Mac is grinding to a halt). It's also inconvenient when you're not at the Mac in question.
Enter iStat for iPhone/iPod Touch. Install a little server on all your Macs, buy the $2 application off the App Store for your iPhone, and the Macs on your Wi-Fi network instantly show up for your monitoring pleasure with that interface I've grown to know and love.
It gets better. Assuming you know how to set up port forwarding through your router, you can also monitor your Macs from anywhere (great for server administrators). You can even monitor the iPhone itself. At least now I'll be watching computer stats rather than playing games.