http://www.aventure.co.uk/
I did this for part of my gap year. Its not particularly cheap, but it will probably be the most amazing thing you ever do. They send you out to super remote villages in wherever you sign up for. They go to Nepal, India, Africa, Mexico, Thailand, a ton of places. You teach English for 3 months and live in a small farming village. You live, eat, sleep, and work with a (usually prominent) family in the village. They feed you and take care of whatever you need, as well as paying you living expenses. Then you get a month of free open travel where you can just go and do whatever you want in the country, and they have a local travel agent book all of your train tickets for you and help you plan it out.
One of the most rewarding experiences of my life. Complete culture shock, you are basically living as a regular person in that region normally would. I lived in a house made out of mud and bamboo, ate mostly just rice and green vegetables and occasionally some cheese or fish. There was no plumbing, you showered with a bucket of water and a smaller bucket to pour over yourself. As well as teaching you will participate in all sorts of rituals, holidays (Indian moonshine lawlz), and ceremonies, and you will likely help with some harvesting/planting as well. I actually got in to the best shape of my life when I was there. Where I was (Himalayas), it was a 30 minute uphill hike to a road or any kind of paved surface, an hour and a half hike to the nearest store to buy smokes or soda or whatever, a 4 hour offroad jeep ride to the nearest city, and an 8 hour jeep ride to the nearest big city/train station. This was the view standing on my balcony.
I can honestly say it was the hardest and most rewarding experience of my life, and I would really recommend it if you can afford it (most of the cost is in plane tickets). I'll probably never experience anything remotely like it ever again.
They don't take just anyone, there is an application and interview process, but they are a British company trying to expand in to America (me and this dude from Vermont were the first Americans to ever go somewhere with the program). So being American puts you at a huge advantage.
I also worked construction 6 days a week for a month in San Diego, made $1600, then went to Europe for 6 weeks and blew all of it. Another experience I highly recommend, although not quite as cool as the other one.
