So you have that fancy new dslr and the kit lens it came with. You have probably taken a lot of pictures with it and are now ready to get a second lens. The best value in both Canon and Nikon systems is to get a 50mm prime lens. A prime lens has one focal length (it doesn't zoom). These lenses are also a great choice because they have a very wide aperture. This wide aperture makes them ideal for low light situations. It will also have the ability to give the look of a sharp subject with blurry background sometimes called bokeh by photographers. In addition to being great optically, working with a prime lens will help you become a better photographer.
$108 on Amazon
$125 on Amazon