Several good quotes I found at Techlaw Journal regarding Bill of Attainder that I thought would be good to share on the thread:
http://www.techlawjournal.com/glossary/legal/attainder.htm
Bill of Attainder
Definition: A legislative act that singles out an individual or group for punishment without a trial.
The Constitution of the United States, Article I, Section 9, paragraph 3 provides that: “No Bill of Attainder or ex post facto Law will be passed.”
“The Bill of Attainder Clause was intended not as a narrow, technical (and therefore soon to be outmoded) prohibition, but rather as an implementation of the separation of powers, a general safeguard against legislative exercise of the judicial function or more simply - trial by legislature.” U.S. v. Brown, 381 U.S. 437, 440 (1965).
“These clauses of the Constitution are not of the broad, general nature of the Due Process Clause, but refer to rather precise legal terms which had a meaning under English law at the time the Constitution was adopted. A bill of attainder was a legislative act that singled out one or more persons and imposed punishment on them, without benefit of trial. Such actions were regarded as odious by the framers of the Constitution because it was the traditional role of a court, judging an individual case, to impose punishment.” William H. Rehnquist, The Supreme Court, page 166.
“Bills of attainder, ex post facto laws, and laws impairing the obligations of contracts, are contrary to the first principles of the social compact, and to every principle of sound legislation. ... The sober people of America are weary of the fluctuating policy which has directed the public councils. They have seen with regret and indignation that sudden changes and legislative interferences, in cases affecting personal rights, become jobs in the hands of enterprising and influential speculators, and snares to the more-industrious and less-informed part of the community.” James Madison, Federalist Number 44, 1788.