There are a large number of volunteers in Kenya, and volunteering can be a great way to reduce the ecological footprint of your trip. As a general rule, volunteering works best for both the traveller and the organisation in question if you treat it as a genuine commitment rather than simply a fun extension of your trip. It's also preferable if you have a particular skill to bring to the experience, especially one that cannot be satisfied by local people.
Keep in mind that there is no such thing as a perfect volunteer placement. Generally speaking, you’ll get as much out of a program as you’re willing to put into it; the vast majority of volunteers in Kenya walk away all the better for the experience.
Note that for any volunteering work involving children, you will require a criminal background check from your home country and/or previous countries of residence.
The following international organisations are good places to start gathering information on volunteering, although they won’t necessarily always have projects on the go in Kenya.
Australian Volunteers International (www.australianvolunteers.com)
Coordinating Committee for International Voluntary Service (www.ccivs.org)
International Volunteer Programs Association (www.volunteerinternational.org)
Peace Corps (www.peacecorps.gov)
Step Together Volunteering (www.step-together.org.uk)
UN Volunteers (www.unv.org)
Voluntary Service Overseas (www.vso.org.uk)
Volunteer Abroad (www.goabroad.com/volunteer-abroad)
Volunteer Service Abroad (www.vsa.org.nz)
Worldwide Experience (www.worldwideexperience.com)