|82 days of uptime on a Raspberry Pi, tracked with a cron job to SQLite database.|
Since the Raspberry Pi uses very little electricity, it's easy to just leave on all the time. As my Pi was on for a while after I got it I realized it started to accumulate some pretty impressive uptime. At one point it had gone 82 days without shutting down or rebooting. Unfortunately, my house lost power due to a storm with heavy winds and my Pi started back at 0.
I'm at 47 days now and attempting to beat my record - there were a few experiments in between that caused some reboots. A friend had his Pi on for 101 days straight when he accidentally cut off its power. A good way to garner good uptime would be to get a UPS for your Pi. They're not too expensive on Amazon and would keep a Raspberry Pi going for nearly 2 hours without any outside power source.
So what's your best uptime? Do you have a screenshot of it? Add it to the comments below. My Raspberry Pi's uptime is accessible on the web here: http://treddell.com/pi/. I'll also update this post if I beat my previous record.