iTunes
Has influenced my lifestyle more than anything else ever on a computer.
No iTunes, no iPod and we'd all be using some convoluted UI on our mp3 players (like WiMP on a iRiver!? - eugh)
Even Pro's have to admit this is probably the best lifestyle app ever.
(I think there should've been 2 categories: Lifestyle and Proffesional)
Photoshop has to be one of the most influential Pro apps of all times too. And the fact that it can be picked up by me, even at release 7, is a testament to how good it is.
Damn it feels good to be a Mac user!