SIL International, also known as the Summer Institute of Linguistics, is the only major organization conducting linguistic field work on an ongoing basis. Most linguists doing field work have the goal of first, documenting the language so that it may be preserved, and second, to discover new things about how language works in general by examining a (usually) yet-to-be-studied language. To this end, they generally publish articles in peer-reviewed journals and present papers at academic conferences. SIL, on the other hand, conducts their research with different goals.
From the SIL website (http://www.sil.org/sil/):
Founded 70 years ago, SIL International is a faith-based organization that studies, documents, and assists in developing the world’s lesser-known languages. SIL’s staff shares a Christian commitment to service, academic excellence, and professional engagement through literacy, linguistics, translation, and other academic disciplines. SIL makes its services available to all without regard to religious belief, political ideology, gender, race, or ethnic background.
The first step taken by SIL linguists is to learn enough about the language being studied so that the Bible may be translated, and most of their work is focused on proselytizing their religion. Though they strive to document languages, it seems that this goal is somewhat secondary to their goal to convert indigenous populations to evangelical Christianity. While some of the members of SIL come out of a linguistics background and do excellent work on documenting the language, which is generally published by SIL International in the form of a grammar, others have very little linguistic training and are under-qualified for the work they are doing, but are drawn into the organization out of evangelical Christian values.
SIL is a very sore spot for many linguists. Regardless of personal faith, linguists generally believe that linguistics is a science, and should thus be handled in a secular way. Furthermore, they have given field linguists a bad name within indigenous communities; I have heard stories of communities, even families, torn apart because some were converted and some were not, and locals have heard these stories as well. As a result, they are very wary about people who call themselves linguists. Linguists are also torn because SIL maintains Ethnologue, the largest database of the world's languages and their relationship to other languages. This, and other resources put out by SIL International, are quite useful and valuable to linguists (both those who work in the field and those who don't), and so we use them, despite our problems with the organization. While most linguists are faced with the difficult task of getting funding from organization such as the NSF, SIL International has large amounts of money from evangelical Christian contributors, making them able to work in the field year-round and gather more data than most linguists are able to during their ill-funded summer field trips.