I like to think that if I had a billion dollars, I wouldn’t waste my money buying myself pointless and expensive things. However, I’m sure I would allow myself at least one extravagance.
For me, I think it would be a huge, high-definition, widescreen television with the best sound system available. Along with it, I’d get the best cable/satelite service available so I can get every channel available in my part of the world.
What would be your one extravagance/luxury?
My own private, customised jumbo jet!
My one extravagance is a 23-inch HD LCD computer monitor. 
Oh, you mean, if I had a billion dollars? Solar-powered houses and a personal space program!
Okay, the space program. I’m already doing solar-powered houses.
A billion dollars, eh? I might spend $24 million on a [url=“http://www.wally.com/jumpch.asp?idChannel=44&idUser=0&attivo=2-6”]118 Wallypower* yacht.
A really nice place to live. Not a mansion, but maybe a perfect-for-one-person house in the city with a big yard for my dog.
That, or I’d use it all to travel very comfortably. Fly first-class around the world and stay in nice hotels and stuff.
I would either a) buy as many books as I could carry every day at Barnes & Noble or b) buy a Barnes & Noble.
I would wear only the most expensive, comfortable and gorgeous shoes manufactured.
Along the same lines as Ellen’s: socks. If I had money to burn, I would wear brand new creamy-soft socks every single day, and give the used ones to Goodwill or something. New socks every day! Oh, the decadence…
Jerry Lewis? 
I would move to England and buy an estate and a title. Then watch what’s left of the British aristocracy twitch and shudder as I introduce myself with, “Hi, I’m Lady Marli! How y’all doing today?”
Either that or me and my friends are going to start our very own commune.
The coolest, most state-of-the-art, comfortable, beautiful, well-furnished, tricked out house ever. And the ability to hang out it in all the time, ie, not working.
I would create a foie zoo. Buy a normal zoo and then gavage (force feed) all the animals to see which ones I could make foie gras out of.
It would have to be a supercomputer that can run/be ruined by running a Windows operating systems so I can play games on a really sweet kick-ass system. And also, at the same time, donate server space to the SDMB.
-FrL-
House of the future, with built-in recording studio full of instruments, state of the art audio system, and the best available copy of every collectible record in the world, to rival even the Michel Ochs archives. Technology out the wazoo, for sure.
I’ve always wanted to be a Marquise…
Other than that, hmm. I’d probably just move into a really, really, really nice house. With lots of bedrooms and a view of Puget Sound. I am enamoured with beautiful houses. Hell, until my life took a recent turn, I was ready to go back to school to become an architect. Maybe I’d build a big house. Or design it. Gimme a hammer.
Damn, I don’t honestly know what I’d do with a billion dollars. Well, I mean, not for myself, as an extravagance. I know what I’d do with it, but never thought of what I’d spend on myself, specifically. I’d be taking most of it back home to my redneck town and investing in education, creating jobs, and cleaning up the town, giving the kids a chance to grow into something better than toothless, drunken, drug addicts.
House. A house. Let’s stick with that answer.
American billion or European billion? 3 zeros difference an’ all, you know…
In any case, it’s so much I’d just have to start throwing it at several charity groups and stuff until I got it to a number small enough to fit inside my brain (mostly people dedicated to health and education), and then I’d buy me a flat in Barcelona (a decent-sized one, right now my price range would mean a 400sqft 1/1 in a basement) and fill it with books and computer.
I might also start a NPO helping people migrate legally to wherever, since I don’t think I’ve ever heard of one. Red Cross, Caritas and workers’ unions help people who are already wherever - I’d like someone to help with the previous steps. Sick of seeing lawyers who know less about the law than I do (I’m an engineer, but also a frequent migrant) get paid for printing out a webpage 
- and getting it wrong!
Yes, I’m pathetic, why do you ask?
“A” Barnes & Noble? Heck, buy the chain!
A Cintiq tablet. Nobody actually needs one, but they’re pretty damn nice to have.
Given how those are only $4000 or so, I’d probably end up donating a good chunk of the rest. When you have that much you stop thinking of it as money in a concrete sense. And what the hell am I supposed to do with a billion dollars, anyway?
Oh, I’d start a business rejuvinating vintage aircraft, restoring them to like-new or even better.
After about 10 years of that I expect I’ll have to get a different job because I’ll have run out of money.