Mac: We've discussed how to make your Mac boot up silently before, but it's a long, drawn-out process. Free app StartNinja will silence your startup chime with just one click.It couldn't be any simpler to use. If you want your Mac to stop chiming when you boot it up, open StartNinja and set the switch to "off". If you want to turn the startup chime back on, switch it to "on". That's it. Pretty easy, huh?
StartNinja is a free download for Mac OS X Lion only. Note that it doesn't seem to work on iMacs, and it won't work if you have headphones or speakers plugged in — but it still solves the popular "oops, my laptop is making sounds in the middle of class" problem.