I can pretty much relate to all these other ISTP gals... I don't know if I differ all that much from an ISTP guy. My interests are probably where I differ most though (as Randomnity said, not really all that interested in cars and such). Oh, and I'm sure ISTP women have learned a little bit more about what "sensitivity" is about than ISTP guys (if only slightly).

(Although my ENFP sister still calls me very insensitive).
When I'm with people I'm comfortable with, I can be wild and fun. I only feel really need 2-3 people I feel close to. As I am very private about my personal life, I may only open up to 1-2 people about something, and they need not be one of my "best friends." (Sometimes I may choose a friend I feel will respond the best according to the subject, and won't even need to tell my best friends)
Similarly to Colors, I consider myself a humorful person and very playful. I'm laid-back, and down-to-earth. It's a little hard for me to adjust in social scenes that have a lot of people, especially if I don't know anyone, but I'm everyone's best friend with a few drinks

Although I love my extroverted friends (And gravitate most towards them), I struggle spending every waking moment with them and doing every social activity they need to do. I -need- time to myself. I like crazy time, and I like chill time. Chill time > crazy time, though. I live moment to moment.... it all depends.
I'm straight-forward and no bullshit when it comes to dating. I -hate- dating games. I don't need to be in a relationship, either. I'm very objective, and can rationalize how I feel about certain things. This quote about ISTPs and love about sums up how I feel about the subject: "ISTPs take their commitments on a day-by-day basis. Even if they say "I do", it usually means "I do for now". They do not like to make lifelong commitments, although they may very well be involved in lifelong relationships which they have taken one day at a time."
What do I do for a living? Well, I -love- problem solving. I graduate in May with a mathematics degree and a specialization in Statistics. I have no idea where I'm going next, nor do I care all that much... I'm gonna wing it.
As for the other 2 questions you asked, Randomnity is dead on when she said:
What are your other interests? What would you do with your life if you were independently wealthy?
nature, travelling, animals, making stuff, exploring places. If I had a lot of money I'd do a lot of travelling, mostly to see things like the rainforest and coral reefs, and I'd like to live in the city with a big yard and lots of pets but have a cottage in the country.
How much responsibility do you believe you have toward others? Is the concept of helping people in general/contributing to good causes high on your value system at all, or do you have more of an "I take care of myself first" kind of mentality?
Not much...I think people should be at least somewhat as responsible for themselves as I am...I've made the decision to avoid donating to charity until I'm out of debt, but I will revisit the issue when I get a 'real job'. I would consider donating to an animal shelter or somewhere I know will spend the money wisely instead of giving the employees lots of money. Nonreligious organizations only.
in more personal terms, I will help friends out if they need help and havent brought it on themselves...ie I would loan money or even give it away to a friend, but not if they make poor financial decisions. There I'd be more like to give "advice", ie unwanted lectures. I expect people to take care of themselves though...that's what adults do.
EDIT-disclaimer: I make no claims about other istps, and I'm not even entirely sure that I fit the istp box very well...
I especially agree with the interests part. Oh, and I'm definitely more likely to donate to an animal charity... I haven't thought of a legit basis for that, but I <3 animals (more than most people)... I feel like most people are more likely to help a charity for people than for animals (which is as it should be, but... someone's gotta help the animals, eh?)