I'm definitely interested in a Martial Art that will teach to defend against multiple attackers from behind. I had a bad experience walking home late at night, so I'd definitely want to learn something that let's me take on people trying to attack from behind.
One of the first things you learn in almost any martial arts (every one I've studied, anyways) and every self defense course is something called situational awareness; beyond that, though, the next step is making yourself a less inviting/tempting target.
When dealing with multiple attackers, the trick is to control their approach so that they can't, essentially, dogpile you. Aiki styles teach a variety of ways of dealing with multiple attackers, some of which are rote and some of which are more freeform. The karate I study also does 2-on-1 improvised self-defenses. The biggest problems with being attacked from behind are (1) you have reduced mobility and (2) you don't really know what, exactly, you're facing.
Taekwondo is more a sport than self-defense. I would rcommend traditional Karate if you like striking.
Tae Kwon Do is, generally, taught as a sport form in the US and even, to an extent, in Korea, but it is, in fact, a complete martial art. I personally prefer karate (the style I got my first black belt in), but I respect the skill and discipline of TKD practitioners as well.
Muay Thai or if you can find a trainer Muay Boran (hardly tought anyware, though some claim they do) are probably the most effective fighting styles. Very effective in that one doesn't need much raw strength to knock out an opponent with your elbow or knee and it's offensive nature means you do not wait for an attack to counter.
Krav Maga is good for training real life situations and teaches some easy and extremely effective techniques to really hurt someone. I wouldn't call it a martial art in the strict sense of the word, but if you only want to defend yourself and not show off to anyone, it's probably the way to go.
I will confess to not being as knowledgeable about Muay Boran or Krav Maga- I've worked with a guy who studied Krav Maga once or twice; I couldn't say how skilled he was, but most of the principles he used and worked with seemed sound. I question the utility of Muay Thai in self defense situations- in competitions, it has not proven to be as effective as traditional karate unless very stringent rules were applied.
It really does depend on your goals and then finding the right instructor. Attitude is very important in your self-defense goals. I recommend you learn the local laws before you consider any lethal action. Even a good hit could be enough to scare away an attacker but really need to consider your goals first.
Holiday makes a good point (something I was discussing with my students today regarding knife and gun laws, actually)- if you are studying a martial arts for self defense, you need to be aware of your local and state laws regarding the use of force. Every state permits the use of lethal force to protect yourself if a reasonable man, placed in a similar situation, would have cause to fear for his life. Once you get beyond protecting yourself, though, and if you have rendered an opponent harmless by other means, then there's a lot more grey area.
