Web Site Monitoring Services?

Napoleon Solo asks: "Unfortunately, it seems like most web hosts still don't do a good job of monitoring whether a server is up or not, especially for dedicated servers. It seems like an off-network monitoring service is a good idea if you want to know when your site goes down. (After all, it's not like your host will tell you). What monitoring services (preferably free) do Slashdot readers like? I'm familiar with QWK.Mon, NetWhistle, and NetMechanic. (Although neither of the latter two offer free service any more) are there any others?"