Cute girls, I'll usually say something to get a reaction, like "I have a technology company. You know, on the Internet." When they follow up with, "Like what?" The response is "Mostly porn." If they take me seriously, they're probably not going to get the real explanation, anyway, so I have a little fun with the porn explanation, making it more and more outlandish until they realize I'm joking. I often tell myself that this is merely for amusement, and that what I actually do for a living is not far more embarrassing (due to being really boring to most people) than running a porn empire.
Other technology people, "I started a company that builds systems management software for web hosting."
Regular folks, "Computer stuff." Very similar to vasudeva's response. The only flaw in this tactic is that every once in a while, you'll meet someone who also does "computer stuff" (or their nephew or son or cousin does), and they'll want to "talk shop" about their website where they sell their knitted dog hats, or whatever MLM scheme they've most recently signed up for that includes a website. Or ask me ridiculous questions about hacking GMail or facebook or something equally retarded (depending on age and demographic).