One of my favorite new tools Are My Sites Up (previous coverage) has a new premium service for its heavier users. $75 gets you some niceties like no ads, secured RSS feeds, direct messages via Twitter, and faster checks on your sites. Users can also add an additional 10 sites to watch on top of the 50 you get with the free level of service.
Coming to the site soon is a mobile interface, statistics tracking, and an IM bot that will send you a message when there's a problem with one of your sites.
Users who are on the fence with this and something like Pingdom might want to keep in mind that Are My Sites Up does not charge anything to send you a text messages, which many other monitoring services do. One thing it's missing however, as some users have pointed out, is that you cannot set it to watch sites on certain ports, which is likely to be fixed in a future release.