Free ways to monitor a domain?
 

I have several domains monitored by InternetSeer because they suscribed me once that I submitted to a SE
They keep sending me messages now and then and say my domains are down, then I ask my hosting and they say they have been up all time long.
Is there any other reliable service that can monitor my domain for free?
I don't need any special features, just monitor for downtime.
Or is it InternetSeer reliable and should I already change hosting?
Any tips?
Thx

http://www.tracert.com/ ?