The double and single-leg shots from folkstyle and freestyle wrestling may be awesome in MMA, but on the street, you really can't use them. I know people usually turn to wrestling for takedowns rather than BJJ, but if it's for a street fight, I'd really look to BJJ for takedowns. The reason being, BJJ was made for streetfighting anyways, so the takedowns work for when guys are throwing haymakers at you. If you haven't seen the Cesar Gracie DVDs, he has a good section on takedowns. Start off by going for the basic bodylock clinch. If he sprawls his hips back, grab the back of one of his legs, and walk your legs forward until your hips are basically under his crotch (this takes less energy than a forward drive and works better), and use your leg to trip his other leg. You can also go around to the side, or to the back. And of course, from that position, you can get a hip throw.
Oh, and by the way, Judo throws will work with a t-shirt. Don't expect to be able to repeatedly do them. But they do work. In Bas Rutten's Lethal Streetfighting DVD, he shows a Judo throw by grabbing Amir's t-shirt and throwing him with it.
But yeah. I'd go with BJJ for street takedowns, but Judo and Greco-Roman wouldn't hurt either. (For street purposes, I'd pick Judo over Greco-Roman, because at least there are foot sweeps and trips allowed)
I see guys hit the double and single leg without hitting their knee to the mat, and it works, but honestly I believe without hitting your knee to the mat, you're taking the science out of the takedown. The entire purpose of hitting your knee to the mat is that you're lowering your body under his hips (the center of mass in humans), and then you're using your leg power by standing up to take him off balance. My wrestling coach, Wayne Griffin, lives, breathes, and sleeps wrestling; no lie. Other than his secondary job as a teacher, wrestling is his entire life. So he shows us some awesome stuff. And when he shows a perfectly executed double-leg, it takes no effort. He uses his leg power and the placement of his shoulder and the positioning of his body to take you off balance. In fact, when we do live wrestling, and he gets a double leg, all he needs to do is stand up to take you down. He doesn't even have to drive.
So unless you have iron knees, I suggest avoiding double and single legs. You're much better off doing BJJ or Judo takedowns on the street. Here's a good list of videos and DVDs for takedowns on the street.
-Vale Tudo Takedowns 2 DVD set by Darrell Gholar
-Ultimate Throws for Vale Tudo by Darrell Gholar
-Judo for MMA by Karo Parisyan
-Mario Sperry Vale Tudo Takedown DVD
-Mario Sperry Vale Tudo Striking and Takedowns DVD
-Cesar Gracie BJJ 3 DVD Set
-Randy Couture instructionals
and surprisingly...
-Bas Rutten's Big DVDs of Combat
^^^ He hands it over to Mansour Heidari; 3 time Iranian national wrestling champion. So even though Bas isn't a wrestler, he makes sure to put a great takedown section in his DVDs.