If you need to monitor any sort of Internet service, or if you want a tool that will watch Web pages for changes, the main game in town for the Mac is Dejal Systems’, now updated to version 3.0. In active development since 2003 and with its first  in five years, Simon 3.0 refines its user interface and extends its capabilities with the concept of filters that enable you to analyze and act on the results of a test in a wide variety of ways. I’ve used Simon for a very long time now to monitor our servers, and frankly, I wouldn’t be without it, since I far prefer learning about server problems from Simon than from someone who happens to notice at some random point in time.
On the interface side, Simon now includes an Activity log in the Monitor window, replacing the old Notifications Log. It still includes the notifications that a particular test has triggered, but also tracks user-initiated actions like edits, pauses, and so on. Also, the editor windows (where you specify tests, services, notifications, and the like) have traded disclosure triangles for tab buttons; a Summary tab summarizes the settings in all the rest of the tabs.
But the most interesting change in Simon 3.0 is the addition of filters. Previously, Simon could determine whether only a particular portion of a Web page had changed, enabling a test to ignore dynamic or uninteresting aspects of the page (ads, the date and time, etc.). Filters abstract that concept, enabling you to look at multiple portions of a page, search for text using simple text matching or grep, analyze found numbers, and more. Filters can even be combined for additional processing power. And if that’s not enough, a Script filter lets you send the results of a test to an AppleScript, shell, perl, or Python script for further manipulation. Much as I wouldn’t really recommend this, you could configure Simon 3.0 to watch a Web page containing Apple’s stock price, and if it drops below a specified level, send a Twitter direct message to your broker to buy 100 shares.
Among the many other improvements listed in are improved handling of cookies, the capability to pause until you’ve logged into a hotspot portal page, support for current Twitter authentication approaches, and the capability to save logs. New services enable you to test to see if a FileMaker Server is running, and to check which applications on your Mac are using the Internet.
Despite all this power, Dejal Systems has a flexible licensing approach that keeps Simon affordable for those who need only a few tests (and note that anyone who purchased since 1 September 2010 is eligible for a free upgrade). The Bronze level offers 15 tests and costs $49 new, or $19 as an upgrade. The Silver level jumps to 40 tests, $99 new, and $39 upgrade. Gold level provides 100 tests for $199 new or $69 upgrade. And the Platinum level allows unlimited tests for $499 new, or $99 upgrade. Simon 3.0 requires Mac OS X 10.5 Leopard or later and is a 15.6 MB download; a trial version is available.