It's funny, most of the ones people list, I didn't find all that exciting when they came out:

* The WWW: "It's so disorganized. I prefer gopher."

* Google: "Well, it gives good results, but Yahoo is 'good enough' for me

* RSS: "Who reads that many blogs?" (I still believe this, BTW; RSS is a technocrats' technology)

* Doom: "Why would I want to play a game where the sole purpose is to blow shit up?"

Things that really were on my "holy shit" list:

* Modems. "I can login to computers halfway across the world."

* VMWare/VirtualPC/SoftPC. "You mean it's like a computer, running inside a computer?"

* Napster. "Wow, free music."

* Gnutella. "Woah, no server, anywhere."

* Processing. "Those are awfully pretty pictures you just whipped up in the last 6 hours."

* GMail. "A gig of storage space. And it's searchable. And it has keyboard shortcuts. And it's got this conversation view. Where's my invite code?"

* Fanfiction. "Wow, hundreds of thousands of people trying their hand at writing stories."

* OLPC. "This'll open up a market of literally half the planet."

* Functional programming. "I'll never make a state error again."