In Adobe Photoshop when you’re designing a site and you need to draw a 1px solid line, I recommend using the rectangle tool (U) rather than the line tool. The reason is that the line tool will leave a blurry edge at the beginning and end of your line even when it’s drawn at 100% zoom. When using the rectangle tool, you’ll have to pay attention to the height of the line but you will get a cleaner result.

–Timmy Christensen