Fly on Desktop. Because sometimes you just need critters buzzing around in your face as you’re working. Yes you do, don’t argue. Freeware (and why would you pay anyway?)
The most realistic fly for your desktop! Fully simulates the behaviour of a real fly. Launch of 5-10 units, and you will believe that these are true flies. The Fly on Desktop has settings, where you can enable it to startup, place on top of other windows and specify the number of flies. To remove a fly from the screen, just double click on it.