Can anyone recommend a good website monitoring service (doesn't have to be free)?

I need sms and email alerts, the ability to send alerts when response time goes up or when a page contains particular text, and a minimum of false positives.

So far I've tested the following freebies and found them lacking in one way or another:
site247 (false positives: otherwise would have seemed the best option)
Mon.itor.us (tests only once a day. Very confusing interface)
Montastic (way too basic)

Next up for evaluation are the following paid services: if anyone has any experience with these, or has other services they think I should try, post a comment below!

siteuptime
websitepulse
alertra
doc-com monitor
internetseer
hyperspin
webmetrics
hosttracker
siterecon
watchmouse

11/15 Correction: mon.itor.us actually tests much more often than once a day.

Amazon has been pushing the limits of distributed computing, offering very useful, reasonably priced computing services like their awesome online storage service (S3), and their queuing service (SQS). Now they’ve released something MUCH more generic and powerful: a hosting infrastructure that lets you preconfigure your desired servers (by giving Amazon a disk image of a Linux machine). It's called the Elastic Compute Cloud, or EC2 for short. When you want a server, you can then order it via the website and have it online within minutes. Pricing is a very reasonable 10 cents an hour (72$/month) plus bandwidth. Each instance provides the computing equivalent of a dedicated system with a 1.7Ghz Xeon CPU, 1.75GB of RAM, 160GB of local disk, and 250Mb/s of network bandwidth.

Yesterday, I found an article on zdnet called "Top 25 on-demand providers". The article did not live up to it's title: in fact, it was remarkably content-free! A little digging turned up the backstory : after ZDNet published the story, the analysts that created the list asked ZDNet to remove the content.

Now ZDNet really shouldn't be publishing content that they don't have a license to. But they shouldn't edit stories beyond recognition, either. If an articles main content must be removed, the best thing to do would be remove the article entirely, not castrate it beyond recognition. Thankfully, for those interested in the on-demand software space, there's a google cache still available with the complete list. The list is also below. Enjoy! [via ken novak]

The major trends in IT today reinforce each other in a powerful way. The two technology trends (Web Services and Rich Clients) are tailor-made for the new business-model trend (On Demand Software). The two technology trends also reinforce each each other, creating a self-reinforcing web of interactions that will accelerate once it gains momentum, and may not stop until it has absorbed most of the software world as we know it!