If you think about what photography is, an art form that uses light to paint an impression into a sensor or film, then I think the best invention is the creation of the FLASH. Having readily available light at your fingertips that can be placed at any angle at any power you want, give your light painting ability a serious boost in how you tell stories through light. If the light from a flash is properly modified, it can blend in with natural light almost seamlessly.
For this reason, I use flash in most of my photos. Flash allows me to paint with light what I want, instead of always accepting what I get. The latter is not as fun. So I welcome the invention of flash with open arms full of excitement. Another modern invention that has changed the game is the introduction of eye detection AF. Now, the photographer no longer must have his/her eye glued to the viewfinder to find focus.