The following are exceptions to the exclusionary rule:
(1) A second, unpoisoned/untainted source had a major role in finding the evidence.
(2) The evidence would have been discovered anyways without the tainted evidence.
(3) Evidence may be used for impeaching a witness on cross examination.
(4) A witness's identification of the defendant is not excluded if the witness could identify the defendant before an illegal arrest (witnesses recognizes the defendant from the crime, not from the line up).
(5) The rule does not apply to evidence presented in grand jury proceedings.
(6) State agents acted under a good faith belief that they were complying with the Fourth Amendment.
There are some others, but these are the main ones. The key is to understand that the exclusionary rule balances the need to deter Fourth Amendment violations and the reality that the police make mistakes sometimes.