Using available technology: I take control of all the Internet’s domain name servers and redirect all website requests to http://www.straightdope.com
Then I take control of the SDMB server and pre-pay for one billion free memberships, charging the amount to Bill Gates’ Mastercard.
Hahahahahaha!
As for me, I’ve been surreptiously breeding and training vast armies of killer squirrels. I’ve engineered them to be cute… cute and deadly.
Oh, good. So now they’re just like they were before, but with cuteness added in.
I just wouldn’t tell the Secretary General about the plans for an Intergalactic bypass. Beuracracy and the Vogoons will handle the destruction.
The Sex Ray from Flesh Gordon. Take that, uptight Earthlings!
I’m not only an evil mastermind, but twisted as well. I’d create a broadcast tasp.
The tasp, as described by Niven, triggers the pleasure centre of the brain from a distance. I would mount my tasp transmitters on satellites, airships, planes, trucks, anywhere I need to irradiate areas of any size. Mob violence? Civil unrest? Celebrity stalkers? Opposing military action? Zap! A blast of orgasm will break the anger cycle and disperse the participants.
The US military has it backward with their agony ray.
I shall create an Army!!!
An Army of…Killer Tomatos!!!
Bwa-Ha! Bwa-Ha! BWA-HA-HA-HA-HA-HA-HAaaa!
I’d try something a little more realistic.
I’d have a 24 hour simulcast of Roberta Gonzalez from KPIX channel 5 doing her weather schtick on every tv receiver on the planet.
If that doesn’t make the U.N. cave in, nothing will.
Given that it has to be flashy, mine would stop the Earth from spinning, either cooking or freezing you depending on which side you were on. Pay up, and I’ll start 'er spinning again 
I’d have a series of three powerful communications satellites set up to flood the world with a single image on every TV and computer screen: Nancy Reagan, naked and masturbating with an inflatable Ronnie.
I figure I’ll get the ransom paid in about thirty minutes.
My doomsday device is fraudulent voting machines. In the 2008 presidential elections, the results will report a write-in victory for George Bush, who of course isn’t on the ballot.
Everyone in the world will immediately reach for the nearest gun and blow their own heads off.
Fantasy: Give me the remastered version from the Star Trek episode “The Doomsday Machine.” I could slice the contries that don’t cave to my demands off the face of the Earth.
Closer to reality: Set off the US nuclear arsenal over Yellowstone caldera, let the volcano and fallout do the rest of the work.
Nicky Tesla’s earthquake machine. It works by being tuned to the harmonic resonance of something. Tune it to that of the Earth’s and it’s death by vibrator, baby!
Comprehend others I mean.
Indeed. As long as they comprehend that it was you making them suffer (for your own fiendish amusement), then I’m down with it.
I’d likely just send out a large number of high-efficiency engines to attach to asteroids of various sizes and push the asteroids into orbits that would intersect Earth’s at approximately the same time. Even if the asteroids were detected as being on collision courses, it’s unlikely they all could be sufficiently deflected.
If I’m permitted to fantasize about my doomsday scenario, I’d also attach an engine to the moon and tell the authorities about it. That would likely keep them focused on my death-by-moon-impact ruse and keep them from guessing my true plan of death-by-multiple-asteroid-impact.
I’d just reverse the polarity on my current plan and bathe the world in Hate Rays. Every single human being would violently, murderously hate every other human being. Every person would do their best to kill every one else. Only the lack of cooperation would keep the planet from becominga huge mass of trinitite.
Of course, my current plan is also of grand scale. When my device is ready, I will flood the earth with Love Rays. Every single human being will be overwhelmed by feelings of compassion and brotherhood. Borderguards will drop their weapons and hug their counterparts. Ambassadors shall suddenly find themselves brimming with trust and negotiating with old pals. I, of course, win the Nobel and am given whatever else I ask for. Being the father of world peace makes for an easy standard of living. Do you see, my fellow Scientists, why the path of evil is so foolish?
Umm… Doc? Are you sure that thing is currently OFF?
Fool! What is easier and faster: self-love or self-lust? My tasp will have them writhing in the aisles before your love ray has time to take effect!