Self cleaning glass DOES exist, kind of.
In theory, glass which is either fabricated with a hydrophillic
surface (i.e. etremey smooth and water-loving) has self cleaning
properties. Glass can also be treated by the consumer of a
professional to be hydrophillic or hydrophobic (water hating).
Here's the catch though...
"Self Cleaning" glass needs exposure to a great amount of water
to actually be self cleaning. If a window that is purportedly
self-cleaning is not exposed to enough rain / water to sheet all
the dirt and debris off the window, then the dirt will remain on
the glass, henceforth, the glass will still be dirty.
My suggestion, unless you want to hose down all of your windows
every few weeks, save your money by buying regular glass and hire a
professional window cleaner to handle your windows for you!