how do you monitor uptime ?

Hi guys !

One of my debian box recently rebooted unexpectingly (which is a bit preoccupying since this is my personal email server among other things).

I still haven't found the cause of this reboot but what worries me the most is that if I didn't stumble upon some message in my logs while poking around, I wouldn't have found my box restarted ...

I currently monitor my servers using logwatch (and the default configuration). I find it very handy, and I like the minimalistic approach. But logwatch does not tell you anything about your server uptime.

Which brings me to my question: what is your way of monitoring uptime ?