Everybody knows that keyboards make sounds when you type. So when you're spending the afternoon reading Perez Hilton instead of working on that report, it's easy to notice the lack of keystroke activity.
We slack at work too, so no judgment from me. If you're hunched over your desk with no typing noises coming from your cubicle, however, it won't take more than a few minutes to figure out that you're just tending to your crops on Farmville again. Idle Keyboard lets you add an audio component to your office fakery, playing a never-ending stream of typing sounds over your connected speakers.
Just launch the site on a tab in your browser and leave it on, as it plays a varying series of typing and clicking sounds in a continuous loop. The sound will stream out of your speaker, so make sure to mute the volume on those Flash games and websites you're surfing to. Don't forget to set the speaker volume at a realistic level, since I doubt anyone will believe your tiny fingers can type like Andre the Giant on steroids.
Of course, if your office keyboards are the silent kind, then Idle Keyboard won't do anything for you. Best get back to work, slacker.
[Idle Keyboard via Red Ferret]