One of my favorite lenses to break out and use every so often is the super wide angle 15mm Heliar attached to a Leica M or or any other M mount camera. The field of view is so ridiculously wide on this lens that when I look through the viewfinder (mounted to the hot shoe on my camera) I actually get a little bit dizzy sometimes. I don't think my brain is meant to process so much information through my eyes at one time! Seriously, it is kind of crazy.
This is a really specialized lens I will admit. As cool as it can be it is also difficult to make an image that doesn't look gimmicky, distorted, or just simply so full of "stuff" in the frame that the final photo is just too messy to be appealing. However, when it works it really works, creating a photograph that shows just how grand this world of ours can be.