Has this ever happened to you? Arriving home with your stunning new widescreen LED or LCD monitor, you set it up and plug it all in, adjust the brightness and contrast and revel in the wonderful display. After a couple of hours trying various programs and games in the new higher-resolution, you open your webbrowser, email client or explorer, and you see a spec of dust on the screen. You dab at it, but it doesn't go away. Then you look closely and realise it's a dead pixel.
Most manufacturers will allow you to exchange a monitor with a dead pixel within the first week, which means it's important to check for dead pixels ASAP. (Incidentally, I won't buy a monitor without a zero dead-pixel warranty. I look at my screen up to 18 hours a day, and the last thing I need is an odd pixel which feels like a grain of sand in my eye.)
That's where LCDTester comes in. Just run it, click Begin and click the mouse to cycle through several solid colours across the entire screen. As each new colour is displayed, check the whole screen for odd-looking pixels. (Usually these tiny dots will be a different colour, or much brighter than the surrounding pixels.)
If you find a dodgy pixel it's time to call the shop and find out what your replacement options are.