Honestly I'm used all the time, but I can't say I particularly care. I have a bad habit of trying to buy my friends. I loan people small amounts of money at lunch all the time without making them pay me back. Part of the reason is because I know how irritating it is to forget your lunch money and not be able to get lunch, but another part is that I want people to like me. So if they know me as the lunch money person, I don't mind, even though I'm sure my handouts amounted to close to $30-$40 in one year. I also buy gum in bulk and hand it out at lunch or in class like I'm Oprah. Everybody loves the gum person. This year, I plan on keeping a little basket full of basic necessities in my locker (pens, pencils, a calculator that I don't use, Q-tips, a bleach pen, period supplies, etc.) so that I'll always have things if people need them.
This also happened during basketball season; I'd bring extra hair ties, shorts, socks, and sports bras (all clean, obviously) to practice and games in case someone forgot. Our first away game of the season I forgot a hair tie and a sports bra, and let me tell you it was super uncomfortable playing that day. I'll always have extra stuff in my locker in case a teammate needs it. Still, I've had people take my stuff and not give it back quite often. I must have gone through at least 10 hair ties and a pair of socks. At least they gave me the bras back.
People also used to use me in class. It doesn't happen as much in high school as it did in middle school because I don't let it happen. My work is my work and there's no way in hell I'll let people take credit for my A+ quality work. I also used to do all the work in group projects, but I gotta say I slid by quite nicely this year. One project in particular, there were four girls in my group and three of us let one do basically 60% of the project (while I did like 25% and the other girls combined for 15%). I should feel bad about it, but she was so gung-ho that I think she really enjoyed it. And we got the best grade in the class.
The only time I mind using people is if they were somehow hurt (mentally or physically) by it. For me, I think this would encompass cheating (in school or sports) to give myself an advantage. I don't do that, though, because I believe in hard work. I don't care about leeching off people for little things, especially when they know I'm leeching and don't try to stop me. That means they also don't mind.
The only time I'm particularly sensitive about people being taken advantage of is if they're incapable of fighting back. I'm fully able to defend myself from being used, but again, I don't mind it much. If someone is afraid to stand up for themselves or don't know that they're being used, yeah, that would definitely steam my broccoli.