Which colours work well together to create a professional looking site? From past experience, blue works well aswell as light grey / silver.
Blue is a very good colour. Its best to have the majority of the site with a white (or very light background), except maybe for gaming site
Light green and blue work together quite well imo. But i think silver (grey) is one of the best colours to use, because it can work with most other colours.
Different shades of the same colour look really good. For example, dark and light blue, or light and dark green. Having gradients (such as the ones this forum does) makes your site look more professional as well.
Use black and grey. Gradient will be good.
Personally, when I want to find out a hex value of a colour to use with HTML/CSS, I use a great site called:
http://www.colorschemer.com/online.html
It really is easy to make schemes too, as on the right is a table of matching colours.