Pingdom also has a very good free plan. It's limited to one url, but for that URL, it's just as good as their paid plans. So you get checks from a ton of DC's, and it tracks response time as well as uptime.
https://www.cloudkick.com/accounts/signup/developer/
CK sent me a text message as soon as my host went down earlier, which gave me time to file the ticket against Linode.
Only caveat - they don't provide a signed RPM, which can cause some administrative hardship for linux users using rpm-based distributions (most of the yum operations require you to add an extra cli option to allow working with unsigned rpm's), but it's a minor complaint given that their agent isn't mandatory and doesn't need a lot of updates.