Sure, here is one way. You are not focussing on the flight at all, but on the volunteer activity you are going to perform at the other end.

So, these pages can get you started How to Take a Volunteer Trip and How to Volunteer and many of those flights and trips they mention are either at cost or you pay extra to donate funds into the project you work for as a supporter of the cause. But really the world is your oyster and much of the travel can be free if you look long enough. For example, the late Oral Roberts ministries Richard Roberts is going to be flying to Kenya for the end of January 2010 with a crusade to feed, heal, teach and minister to the people of Kenya just like his father did. They were sending over almost 2 million meals to go with it as well as medical teams to the second largest slum in the world to help, because there has been no rain for two years there and it is a desperate situation, so they may need volunteers to feed people or to help organize it although it may be a bit late to sign up. They only needed one more doctor they mentioned this week to work in a clinic free that will see 7,000 patients a day. In that case, if they accept you for an event like that as a volunteer, your flight may be booked by them and not by you at all. Their url is here http://www.orm.cc/ based in Tulsa, Oklahoma. They are still looking for $50,000 to buy another half million meals to take over because the need is so great once the food arrives it is gone and they want to bring more to feed the audience, and before that they were still looking for shoes I think. The place they are going to is the size of a city.

Also shows like 100 Huntley Street often feature guests who need volunteers in all parts of the world and they also send mission trips for sustainable projects like digging wells and distributing school supplies etc. They have a Youtube channel that you can watch snippets of shows from here http://www.youtube.com/user/100huntley , maybe they will have the segments where they ask for volunteers on there and you can see who leads or participates in each project, the main website is here http://www.crossroads.ca based in Canada. Almost every religious based show on television likely has some connection to or knows of volunteer opportunities from every country helping other countries. Hillsong TV is an Australian show shown in over 200 countries, they are based from Hillsong Church and their music division includes chart topping Hillsong United and they also raise funds for and go to Africa to help people.

If you are interested in flying with a more specialized group here is just one example of maybe hundreds of thousands of choices I would guess: http://www.soles4souls.org/ Soles 4 Souls sends donated shoes and distributes them worldwide. If you click on Ways to Help, you will see the button Distribution Trips. You can pick your destination and itinerary and they will send you there free I think - you can check, to distribute shoes http://www.soles4souls.org/areas/trips.html .

Of course these organizations often work together, so when you arrive in Kenya with Richard Roberts and I don't know this, but you may very well find that the donated 8,000 shoes you will be distributing originally came from soles4souls! It is an entirely different network for travel, and can be free also or less than cost, but it definitely can be classified as life-changing. Once people take such a trip, often they return again and again with their children and friends to different parts of the world - that is what it mentioned on one of the shows. And the children are influenced by it, and remember it all their lives and may also choose to enter a career that helps other nations so helping others can find you a new job and career also. Is that different enough for you?

Do not fly friday/saturday/sunday. I have often gotten better rates flying tuesday to tuesday,