[Editor’s note: The following review is part of Macworld’s GemFest 2009 series. Every day until the end of June 2009, the Macworld staff will use the Mac Gems blog to briefly cover a favorite free or low-cost program. Visit the Mac Gems homepage for a list of past Mac Gems.]

Digital Sentry is an aptly named program that monitors your computer for any of a number of events. When it detects one of those events, it then takes whatever action (or series of actions) you’ve defined for that event (Digital Sentry calls a defined set of actions a monitor). What sort of events can Digital Sentry detect? A surprisingly wide variety, including the passage of a set amount of time, an application being launched or activated, a mouse button or scroll wheel being moved, a change in power source or screen resolution, a volume being mounted or unmounted, and a user password being entered incorrectly.

Once you’ve chosen an event to monitor, the next step is to define the action or actions to be taken when the event occurs. There are 16 actions to choose from that cover the expected (run a shell script, open a file or program, put the computer to sleep) to the unexpected (display a fake security alert, speak some text, take a Web cam picture, send a text message). Each action will have some associated configuration options, such as the text to speak, the amount of time to delay, which file to open, volume level, etc.

Actions are added by dragging them from a source list into a blank work area, much as workflows are built in Automator; you can also rearrange actions by dragging them around within the work area. A simple example would be a monitor that waits a set amount of time, then runs a backup program. Or consider a scenario where you suspect someone’s been trying to break your password on your cubicle-dwelling Mac at work. Using Digital Sentry, you could quickly create a monitor that does all of the following:

• Take a picture of the person with the iSight camera and automatically e-mail it to you
• Send your phone a text message
• Audibly tell the intruder to go away
• Put the computer to sleep, or shut it down

The possibilities are limited to what you can think to link together—and with the ability to execute shell scripts and run applications, you can make your Digital Sentry monitor do pretty much anything. When running, Digital Sentry is a faceless background application; it doesn’t have an icon in the Dock. All interaction with the program starts with a click on its menu bar icon. You can also set a password for Digital Sentry, so that it can’t be accessed or quit without supplying the password.

The interface for creating monitors is nicely laid out, making it easy to create a new monitor. The only thing that’s a bit odd is that you have to name the monitor before you specify the event you’d like to monitor. If you aren’t familiar with the available events, you’ll probably end up skipping forward to see the events list, then returning to name your monitor. But this is a relatively minor quibble.

Digital Sentry has one final feature that’s likely to be controversial: a keystroke recorder. Accessed via the program’s preferences, the keystroke monitor will record (to a log file) everything typed while your user is logged in. In my testing, the recorder captured almost all my keystrokes. When I entered my password to unlock the screensaver, this wasn’t recorded; a password entered with sudo, however, was captured. On my machine, at least, there were also many extra characters recorded, especially when I used the Shift key. A capital “E,” for instance, appeared as “AE,” and was occasionally followed by an “a.” Thankfully, Digital Sentry doesn’t try to hide its existence; if you can see it in the menu bar, you know it’s running.

Digital Sentry is a unique application—I’ve not previously run across anything that does exactly what it does. Documentation is a little light, but there’s enough there (accessed via the Digital Sentry Help entry in the program’s menu bar icon) to get you going.

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[Rob Griffiths is a senior editor for Macworld.]