Walter Pincus has an excellent story which organizes all the known facts regarding who sent Wilson to Niger.
This has been a contentious issue but mostly for those trying to score political points. It has been raised most often by conservatives attempting to discredit Wilson's criticism of the administration.
The CIA and Wilson maintain that while Plame was involved in process, Wilson's name was chosen by others. Defenders of the administration's use of the uranium story (since proven incorrect, as Wilson's critical New York Times op-ed stated) claim that Plame sent her husband in order to discredit the administration.
What can be said for sure is that Plame did not have the power to send her husband and Wilson's criticism of the administration's use of the report he put together was proven correct very quickly. In fact, CIA Director George Tenet had his hand forced and took responsibility for the inclusion of uranium procurement claims even though the CIA and State Department worked hard to keep the claim out of the President's State of the Union speech.
Pincus' piece breaks down the timeline as well as showing that there is some good evidence that Plame, in fact, did suggest her husband for the trip despite Wilson's claim that he was picked by others.
Certainly administration defenders will jump on this, but the question of who sent Wilson is immaterial. Wilson was the last U.S. Ambassador to Iraq and had contacts in Africa and was well regarded by President Bush's father, Herbert Walker Bush. He was well qualified to lead such an investigation.
Wilson went public with the fact that his trip was being manipulated to support false claims and he and his wife were attacked as a result. They were attacked for being credible critics of the administration's fabrications.