A few months ago I read this post on Brandi's blog about the new lens she'd received for her birthday. It was a 50mm f/1.8 fixed lens, and it seemed to produce a rather lovely shallow depth of field. I'm a sucker for shallow depth of field photos - where one point is picked out in crystal clear focus and everything else fades into a delightful blur - so the lens duly went on my Xmas wish list, and my awesome Mum duly went out and got it for me. And I am so very happy that she did.
The low aperture is responsible for the super depth of field, but it also means that the lens sucks in all the available light it can. This means even in low light conditions you can have a fast shutter speed, which makes it great for candid portrait shots as you don't have to get your subject to pose. The blurry background also leads to happy bokeh accidents, as I'm just starting to discover.
If you want to read more about why the 50mm fixed lens is ace (and how it will make you a better photographer) this article is a good place to start.