Any PINGDOM Open Source or free Alternatives ?

Dear all,

Do you know any Pingdom alternative ?
They charge a damn high amount for such a simple service.

I'm using some "low cost" hosts and I really need to be able to monitor the downtime !!!

I'm pretty sure there must be an alternative to Pingdom but I couln't find anything. I'm thinking of a script running on a "reliable" host pinging the "unreliable and/or lowcost" host and sending alert email via SMTP in case of downtime...

Any idea would be GREEEATTTT !

Thanx !