Horizontal scrolling web sites were once considered bad practice, but I believe most will agree that for some purposes e.g. for showcasing a portfolio it can be a quite useful and attention generating approach. While researching for this is post I found that in most cases the horizontal scrolling have been put to use in an intelligent way and user friendly way. Still keep in mind that horizontal scrolling is certainly not good idea in every situation, but when done properly they can really make a website standout.
Common practice today is that web sites use the vertical direction to give place for content that does not fit into the screen, i.e but allowing readers to scroll down. This is how it has always been and we are so used to this approach that it will be difficult to change. It is interesting though to consider the navigation design in iPhone and iPads where a lot of navigation actually goes in the horizontal direction. It works really well and users love it. However in most apps reading text and web pages still require vertical scrolling. To give you an idea of how horizontally scrolling web pages work I have collected 35 excellent examples of websites making use of horizontal scrolling for main navigation.