Windows only: Open source program Ghostzilla is a parasitic browser based on Firefox capable of embedding itself inside any program on your Windows desktop for inconspicuous browsing.
Ghostzilla works like this: You run it, and Ghostzilla embeds itself in a frame of the current active window. For example, in the screenshot above I've got it embedded in a program I never open, Outlook Express. Okay, that's cool, right? But it gets even better from there.
Ghostzilla makes your browsing as inconspicuous as possible. Everything in the browser window is gray-scaled, images only load when you hover the mouse over them (though you can change these settings if you don't like them).
Finally, if you want to quickly "ghost" Ghostzilla, just move your mouse outside of the program's window. Ghostzilla disappears, and it won't reappear until you move your mouse from the far left of your screen to the far right and then back again.
The developer of Ghostzilla suggests that you don't use it at work (he doesn't want you to get fired!). On the other hand, having once spent the better part of a year at a job where I was only assigned two hours of work a day (at best), I urge you to use Ghostzilla!
Obviously I don't want you getting fired, but if you're the type who stares hopelessly at the wall because you're too nervous that someone might catch a glimpse of you looking up a recipe for dinner, I say go for it. No one will be the wiser.