There’s been so many… and most of them have good and bad points. And for me to call something “the greatest invention”, it’d have to have no horrible sides to it.
Personal Computers — great, they’ve helped probably in more ways than hurt, but (yes) they’ve hurt. Think of all those families who’ve had problems due to computer-related addictions — gamers, internet junkies, etc.
Cellphones — great, only have one and no land-line, but they’ve also hurt the way we interact with each other as a society.
World Wide Web (no, the Internet pre-dates me) — again great, it’s how I’ve put food on the table almost completely since college. But again, it’s turned into a monster by reducing the direct inter-personal interactions that once were the basis for our great society.
Digital Photography — good, love it, it’s actually *helped* me learn more about photography than I did with film. The notebooks of camera settings weren’t ever able to be paired up w/ the shots in the film days due to lag in developing… and forgetting what I was trying to achieve. Digital it’s all in the file, so I can check instantly and the record is there later. But… digital has pretty much killed film, cheapened the act of photography (even though equipment costs are HIGHER than film), (again) hurt inter-personal connections by making people impatient, and hurt the world by allowing untalented hacks to flood the market with low quality crap, thus lowering expectations, raising skepticism, and making magazines nearly worthless due to the lowering of standards that are accepted.
Digital Audio Formats (e.g., MP3, MP4, & WMA) all have increased people’s ability to listen to music. They’ve made it possible for people to quickly spread music and have allowed those in the music industry to be heard who would probably have never been heard without digital audio. However, the ease with which files are copied and shared between people has hurt the industry. Songwriters don’t make that much as it is, and it’s even less when people are “sharing” music w/o buying the albums (or tracks). Without songwriters, there is no music.
Self-stick stamps are nice, so are the same for envelope flaps, but with the proliferation of electronic communication and rising postal costs, who uses those much any more. When they first came out, I would’ve called them “the greatest thing since sliced bread”. But not now… not really.
Honestly, I can’t think of something that I’d call “the greatest invention of my lifetime”.
My favorite (at the moment) is Kodak’s Endura Metallic paper. 
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