Simon (from Dejal Systems) is a reasonably priced, feature-rich site-monitoring tool, especially useful to administrators who need to keep track of multiple sites, servers, and applications. The app's primary interface is a straightforward Monitor window, which displays everything that you're tracking with stats on uptime, time until the next check, and time since the last change and failure--along with graphs, lists, and more details when you drill down. You can drag and drop URLs onto the Monitor window, or import from bookmarks, and Simon's services let you define what you want to test, from SMART status to mounting drives, communicating with mail servers, checking and relaunching apps, and more.
Simon is even more useful outside of its main interface: you can access many functions through the Dock or a Status menu in the menu bar, and you can receive notification a variety of ways, including via Growl, speech, Twitter, or e-mail. Simon can even send a text message to your phone when a site goes down, or just log the event in a database. You can modify all these notification options with plug-ins or scripts (including AppleScript and shell scripts), and Simon also lets you customize its services, allowing you to create new ones or simply edit the defaults.
Advanced users will appreciate all this customizability (along with extras like session capturing and multipage reports), but even less experienced users can quickly get up to speed with this flexible, reliable utility.